"surprise/forgotten rules" affecting the fun

[Garwain]

Sometimes the DM recalls (or worse: invents) a particular rule that we were neglecting the whole time and suddenly wants to enforce it right away. Sure enough, it changes the course of the battle, usually in the DM's favour.

Should he not using the 'forgotten' rule untill the combat is over, or use the rule from that moment on?

The second option seems logical because it is only rectifying a wrong situation, but I feel it kills the fun as the players could not anticipate. Often we come to the agreement to continue as before, because "we've been doing it like this, so let's go on like that". But then we are willingly dismissing a legal rule.

[dsmiles]

Sometimes the DM recalls (or worse: invents) a particular rule that we were neglecting the whole time and suddenly wants to enforce it right away. Sure enough, it changes the course of the battle, usually in the DM's favour.

Should he not using the 'forgotten' rule untill the combat is over, or use the rule from that moment on?

The second option seems logical because it is only rectifying a wrong situation, but I feel it kills the fun as the players could not anticipate. Often we come to the agreement to continue as before, because "we've been doing it like this, so let's go on like that". But then we are willingly dismissing a legal rule.
Personally, if I, as a DM/GM, forget a rule, and suddenly remember it during a combat, it will take effect at the end of that combat.

Also: Willingly dismissing a legal rule is not always a bad thing. It removes infinite wishes and chain gating solars/efreeti.

[Eldan]

Yeah. Rules as Written are a horrible mess. If it doesn't cause and gigantic problems, just leave it. Otherwise: never change rules in the middle of the action.

[Epsilon Rose]

If it's actually forgotten I'd leave it up to the PCs on a case by case basis.
As for making up rules... I had a dm who did that once. It was infuriating. If you really want to change the rules I wouldn't do it until the end of the session and I'd give the PCs the option to change stuff in accordance with the new rules (again this doesn't apply if every one agrees to change it immediately or if it's to stop WotC game breakers [see infinite wish]).

[Angry Bob]

Unless it's grievously affecting the session, I usually wait until the end of the session to change the rule, or look it up at all.

I also have the concession that if a rule or houserule changes a character noticeably for the worse, the player can rebuild that character for free.

[Ravens_cry]

A DM, in my view, should make a list of houserules they know they use at the start of the game. After all, unless you're playing characters from their birth, the characters will have some idea how the world works, even if the players don't, and will make choices accordingly. From a players perspective, customization choices depend on knowing what they do. If you make it so the 3.X feat Weapon Focus gives a scaling bonus by level, or that you make it so a Wizard being hit disrupts the spell automatically as in AD&D, the players should know as it will affect their choices and actions.
Simply springing it on them feels arbitrary and is rather unfair in my opinion.
As for forgotten rules, in general I think it is better to stick with the unintended houserule unless the houserule is screwing something up or making things non-fun. Having the universe change around you breaks verisimilitude, like in the very first Order of the Stick strip. If you do change it, I think it is only fair to allow some retraining on the affected players, as no one likes to feel useless.

[huttj509]

Now there can be things that seem like changes in the middle of things, but were actually planned.

"What? Why didn't that affect him?"
"Good question, you can look into that back in town/tower o' lore/etc."

If, for example, the BBEG, having seen the PCs using the same tactic against minions/lieutenants, made a special item to counter it, now the PCs need to try something new or find how to nullify that item.

I mean, being conveniently immune to pc abilities all the time is easy to overdo, but it can be plot interesting if done well, and reasonable within universe. Especially if the BBEG's genre savvy.

[TheCountAlucard]

While this question is largely system-independent, I'd like to point out that for some systems, this sort of thing is more lethal than it is in others.

For instance, let's go with, say, knocking people around. In D&D, it's assumed that nobody's actually going to get sent flying from an attack unless it's a bull rush, which is an option one can take in lieu of an ordinary attack (barring certain other stuff). It's usually fairly-harmless (barring certain other stuff), so it's relatively-safe to forget about, and while bringing it up when the PCs are dueling with an Ogre Barbarian atop a series of small platforms can be a game-changer, it probably won't be any more lethal than just having the ogre attack them normally.

For Exalted, on the other hand, knocking people around is one of the base assumptions; get dealt more damage in an attack than a certain threshold, and your character goes flying. There are even abilities that make getting flung around automatic. Forgetting about that sort of thing only to reintroduce it in said small-platform battle may well kill unsuspecting PCs.

And the "GM Interpretations" bit, too. (grumbles about Black Mirror Shintai"

[Kylarra]

To be fair, if your PCs are letting people hit them for enough damage for knockback, they were probably not going to live long anyway.

[Tyndmyr]

Sometimes the DM recalls (or worse: invents) a particular rule that we were neglecting the whole time and suddenly wants to enforce it right away. Sure enough, it changes the course of the battle, usually in the DM's favour.

Inventing rules mid-combat = terrible.

If anyone, player or DM recalls some circumstantial benefit(hey, I forgot about buff x) or the like, they are welcome to use the modifier from that point forward in my games. Going back to redo what happened three rounds ago? No. Shoulda remembered then. That seems reasonable.

Changing the entire combat mid-way through? Not so much.

[Cyrion]

Changing a rule mid-combat (or possibly mid-session, depending on the situation) is a definite Bad Thing.

If you've forgotton something or realize that you were applying a rule incorrectly, let it go until it's convenient to talk about it with the players and allow them to potentially adjust accordingly.

[valadil]

Honestly it depends on the system and the encounter.

If I'm running an underwater fight and suddenly remember that the players should have been moving at half speed, I'm going to enforce that rule from then on. It was part of the premise of the fight. Even though it's my mistake, excluding that rule kinda invalidates the whole thing. I feel like the premise of a fight should be honored in spite of my inability to remember rules.

If adding in a rule like this screwed the players, I'd probably give them a round or two without the rule to adjust to it. For instance, let's say there's a catwalk over a volcano. Three rounds into the fight and I remember that they should have been making balance checks all along. I could argue that the players are at an advantage since I effectively let them succeed their balance checks for free for those rounds. But they went farther out onto the catwalks than they would have if they knew they'd have trouble getting back. So I'd tell them I made a mistake and that if they want to reposition themselves to a safer spot, they can do so this round without rolling and then after that balance checks will be needed. I think that's fair. Or I'd improvise around the mistake and say the ground starts shaking and from now on balance checks will be needed even though the ground was stable before.

But if I've forgotten or misread an individual creature's abilities I let that slide. It doesn't matter if it helps or hurts the players. I used some trolls a few weeks ago and forgot that they could get back up if they dropped below 0 HP. For the players who memorized the monster manual, this made the fight interesting. Even though they were weaker than your average troll, these trolls were special and memorable. On the other hand, last session I misread "reach 2; targets..." as "2 targets". I don't know how I dropped a whole word there, but suddenly my brutes were hitting two different targets and the fight was damn hard. I can't even tell you how many times I've misread burst and blast.

I figure changing things like that around, even if unintentionally, keeps the encounters fresh and unpredictable, so I don't correct forgotten rules in that case. That only goes for 4e though. Its combat seems resilient enough that I can misread things and it won't ruin the fight. I'd have to reevaluate how tolerant of misreading another system is before sticking out mistakes like this.

[eepop]

There are other times though that it can seem like there is a "surprise rule" that its more a matter of the DM forgetting something he consciously took into account when building the encounter.

I have a bad habit of getting caught up in the outbreak of an encounter and forgetting that I built the encounter with X thing in mind. The enemies go once without taking advantage of X, then before the next turn comes up I realize I forgot about it and want to start doing it. After all, the whole encounter was built with it in mind, leaving it out very well make the fight a cakewalk.

[Tyndmyr]

Well, if it's something simple like "he forgot to chug his potion before combat"...no biggie. He can chug it now if it still makes sense to do so. If not, well, more loot. Oops.

If it's something such that there is no possible logical way he would have not used it already...don't bring it in. If it's something where you have to explain that you forgot it earlier so it makes sense to the players...don't.

[Epsilon Rose]

While this question is largely system-independent, I'd like to point out that for some systems, this sort of thing is more lethal than it is in others.

For instance, let's go with, say, knocking people around. In D&D, it's assumed that nobody's actually going to get sent flying from an attack unless it's a bull rush, which is an option one can take in lieu of an ordinary attack (barring certain other stuff). It's usually fairly-harmless (barring certain other stuff), so it's relatively-safe to forget about, and while bringing it up when the PCs are dueling with an Ogre Barbarian atop a series of small platforms can be a game-changer, it probably won't be any more lethal than just having the ogre attack them normally.

For Exalted, on the other hand, knocking people around is one of the base assumptions; get dealt more damage in an attack than a certain threshold, and your character goes flying. There are even abilities that make getting flung around automatic. Forgetting about that sort of thing only to reintroduce it in said small-platform battle may well kill unsuspecting PCs.

And the "GM Interpretations" bit, too. (grumbles about Black Mirror Shintai"
I'd like to point out that there's a huge difference between forgetting about an option and forgetting about a rule.
For example, if you just remembered the ork can bulrush it's fine to have him do so because there's no reason he had to be before and no reason he can't now (unless the players some how found out he couldn't) but it wouldn't be okay for you to decide that bulrush isn't strong enough so from now on it'll do double weapon damage instead of no damage.

[Jay R]

I won't play without sufficient information.

No, that doesn't mean that the DM is doing something wrong; it means that until you tell us what rule was remembered in the middle of combat, and what the situation was, it's impossible to form an opinion.

I can invent scenarios in which it is horrible, and other scenarios in which it is crucial.

What's the actual situation?

[Bang!]

I won't screw the players in a tactics game. I'll let the thing play out, then bring up "you know, I think being underwater blocks line of effect" and announce a decision on whether to use it in the future.

But this is one of my big complaints with rules-heavy systems. Having to remember a thousand pages of rules to make-believe is hard.

[nedz]

Whilst I do publish the campaign houserules up front, and debate all rules changes with the players - only applying such changes with a consensus, situations can arise when an obscure rule first emerges.

In such cases I am quite happy to stop and work out what happened. Such instances are quite rare.

You can however get contradictions with many rulesets; which can result in a kind of Xantos Pileup, if many rules collide. This, in itself, can be fun - once in a while.