This stuff is always about bad execution, I think.

My worst DM story hinges on stuff that could have been awesome, if it wasn't all done with the express purpose of railroading and trying to prove social dominance over the other players (yes, players, not characters).

We had a game start with the lines "this is a big new world, and you can leave your mark on it! I look forward to whatever changes you all cause." It ended with "I didn't really want you to try and change the world, I meant discover all the stuff I made!".

Our first encounters were in a dungeon crawl. The DM set up goblins with wands of exploding spell fireball that were being dual wielded. He wanted to explode the two most experienced players into a pit to be eaten by carrion crawlers, in order to keep the other players in line (his justification, might I add, not mine). This ended with the players having wands of exploding burning hands!
"Oh, uh, the exploding was from a feat. You don't have it. No knock back."
Okay. How were they dual wielding without the feat?
"My game doesn't use that feat."
Oh, okay. So we'll dual wield these wands–
"You need a feat to do that."
You just said we didn't? – new player
"Oh, uh... Each wand only has two charges left."
That worked out fine.

The dungeon ends with the goblin boss – "a very tall elf-like man with huge, black armor and two long swords". Man, that's like, straight out of the description box of your old favorite PC that you would never let anyone kill!
... Oh.

So we have to sign contracts of servitude in our blood. Experienced players signed using spattered goblin blood, so we got away okay, but just barely. Good rolls, and we had the DM roll publicly else he would have fudged and thrown off the new players...

First thing we do? Fight to the death to see who is most worthy to serve!
So began my very brief and very disappointing love affair with dagger spell stance. It lasted four rounds of stubbornness before I admitted it did suck as much as the other player said it would. We got out at around 1-3 HP on everyone. After a little adventuring, we decide to talk to the guy about not working for him. I mean, no way to discover a whole new world while shackled, right?
So we walk into the room in the dungeon.
"The BBEG turns to you."
Okay, I now politely and—
"Power word: kill. Roll initiative."
The other player died, and I had dimension door prepared so we got out. The party Druid got him back o. His feet through some clever fast-talk to grant lesser reincarnation as a Druid spell at lower level because it wasn't over-written in the 3.5 update, and we all fled.

Every six hours, random assassins would show up. We avoided them, but it was clear they weren't there to hurt us, just stress us out. And next town we get into the mayor apparently hates elves and so won't serve us (even though we had two elves in the party and kicked them out). Actually, it turns out he doesn't hate elves, he hates us! Because we talked back to his son. We explained the situation, apologized, and went to an inn.

The inn was barricaded and flooded with flying wizards who had greater invisibility up and were lobbing fireballs at us. Superb use of the battle grid got us mostly out without a hitch once we get back into teleport formation due to obsessive observation of enemy cycles and initiative counts. We end up in the basement of a nearby ally establishment. The DM informs us that the rogue managed to grab a sack off a flying invisible wizard somehow, and he rolls with it. It's full of money, and also a massive jewel. I panic and run to the furthest possible corner while warning my comrades, who just look at me funny (except the sorcerer, who also books it). Turns out it was a disjunction trap? And the DM advises the party to all make DC 45 saving throws or we lose the ability to cast spells forever because that's what dysfunction does. He is smug.

Luckily, everyone flipped to the spell to figure that out and deduced that he skimmed the spell and didn't read it; he was going to be insistent on Dysjunction stripping magic permanently in his games until he realized the experienced players (wizard and sorcerer) weren't in the blast and we would be throwing it back at him post-haste. We were rapidly approaching level six by this point. I finally finish the magic item I have been working on; a hat of nondetection. Just in time, too, because from now on every six hours we get targeted with Scry and Fry centered on me. Good thing i got the Dm to okay magic item creation but couldn't get his attention to explain what I was making!
We of course, get numerous events like a stray wind or passing bird knocking off my hat and then the divination assassination begins again in earnest.

At level eight we get into a dungeon the DM led us to that is three rooms, the first of which is trapped to lock us in, block teleportation, and allow 1d4 mummies into the room every 1d6 rounds until all sixty four mummies have emerged. The ranger saved us with his lazily explained multimanyrapidshot shenanigans that shouldn't have worked, let alone with his cheap custom incendiary grenade arrows he had been building with DM permission. I would hazard it was on purpose but the DM didn't know mummies were vulnerable to fire. His stated reason for the trap was to kill the sorcerer and wizard.

By level 13 the party Druid is a demigod. The party fighter has a shield of regeneration and a legion of death knights. The experienced players are wanted in every town and city and patrolled by absurd monsters. We had left the rails at this point, so we are harassed by a log.

A piece of wood? Part of a tree? A log. Turns out the log is a fallen god epic wizard/epic sorcerer's familiar who is guiding us back to the prophecy we never heard of and no one could tell us about. It was here I began to lay the seeds of my revenge.

I got a hold of a scroll of arcane Genesis. I built a prison of timeless agony. All my spare time was spent carving runes and sigils of pain, suffering, madness across it's entire surface. Any off-hand super magic item was disposed of here, fabricating adamant mazes with mystic back tracks and confusions, that were alive and shifted. Golems and constructs patrolled it's interior as guards. Legions of undead guarded the golems. Every trick I could conceive of got brought up enough for the DM to say "yeah yeah, sure, whatever" and noted meticulously on a ledger.

We got wishes. Subtle ones, only. So I wished to change my creature type. I retrained my class. I spent time devising new spells. I kept items with only single charges remaining. I cast no spell above second leek in combat; my contributions were Mage armor, shield, and magic missile. I take notes. I watch, I wait. We are 16th level.

Our enemies? Everyone who ever screwed with us. Literally. The town mayor? 20th level wizard. The innkeeper? Same. The farmer we wouldn't but turnips from who harassed us in town? Same. The librarian who stole the ranger's girlfriend? Same. And in their midst, the Tall Elf with black armor and two short swords, who was about to become a demigod incarnation of demogorgon, which is apparently the plot?
"You'll have to watch him ascend and then fight his god form"
Oh no. I have had enough of this. We can see through this scrying lens that more cultists are teleporting in. We teleport in.
"Oh no. They're plane shifting, all teleports are blocked."
Fine. Fighter! Use your cross class UMD to read this scroll on initiative count 2. Sorcerer, use this staff with greater plane shifting on initiative count one. Fighter, don't start until we get to heaven! Everyone, battle formations!

So we enter the cult mid-ritual. An anti magic field drops immediately, meaning it's melee or bust thanks to excessive use of Widen spell on the scroll prior. Unfortunately one of the cultists was the king of all red dragons. Which the DM had set up legitimately and we hadn't bothered checking for polymorph...

We win on a technicality as the party rogue determined that the BBEG's resistance to poison is, in fact, magical and not inherent. We survive 20 rounds of melee against numbers beasts because the DM rules the divine nature of the Druid (being a god) creates a bubble i the anti magic zone that the fighter uses to maintain his regeneration shield. Eventually, the rogue lines up a shot and hits the BBEG with a couple dozen doses each of int, wis, and Cha draining poisons and positoxins, flooring him instantly.

The DM starts a CUTSCENE! Gods descend, gods rise, the two fallen god brothers start to claim and maybe rehabilitate the BBEG and—

NO.

Time stop. Previously established as stopping even the gods, except the BBEG who had the epic feat to ride along; that is, only he and I were there.
I cast eternity of torture.
"He's immune to wizard magic"
I am actually not a wizard anymore, having shifted to bard/sublime chord/seeker of the lost arcane arts/ultimate magus.
"He's still immune to mortal magic"
I'm my mortal, having become a draconic get resonant image cast by multiversal principles through the last six type shifts from wishes and rituals.
"He has massive spell resistance"
Which is, by your houserules earlier, not able to protect him from this spell.
"Fine! But once the time stop wears off—"

The kicker.

I cast the epic spell eternity of utter damnation.
"WHAT?!"
I cast this epic spell I've been working on that you okayed.
"You're not epic level!"
I'm draconic, have a true dragon age limit, and old enough dragons qualify for epic.
"You're not old enough!"
I've spent months to years e'ery night for six years of game time in a different dimension, crafting and planning.
"You don't meet the skill requirements!"
Except for your houserule that lets is train really hard to go above our maximum rank limit with a high cost.

I threw every loophole, every asinine mistake, every idea he ever forced on the party despite polite discussion and even pleasing, back in his face. He ended up with his most beloved PC ending, not as an over deity, but as a nigh-lifeless husk suffering beyond humans imagination for an eternity of eternities, in nested realities, only accessible through a single jewel around the neck of the over-goddess we had unwittingly powered up this entire time and who was the only being who actively disliked the BBEG.

Two years of being prevented from the simple goal of having fun adventuring being thwarted, collapsed into a sucking wound of malice in the fabric of his games reality. We gave him anther chance later on, in a game where we switched DMs every session. That was six months of actively undoing anything someone else adjudicated so he could start a railroad before we called it off.