Dysfunctional Rules X: I Cast Comprehend Rules

[ShurikVch]

Something I was surprised not to see on the handbook. Players guide to Fearune lets you take regional feats you would otherwise not qualify for if you take 2 ranks in Knowledge Local at level 1. This lets you, among other things, take Jotunbrud on a non-human.

That's incorrect: Forgotten Realms Campaign Setting allowed it; no such permission in the Player's Guide to Faerûn...

This does a lot of silly things but my favorites are letting you be a Kobold that counts as 3 different size categories when it would be beneficial, allowing you to be a small tiny at large, and letting you grapple a Goliath.

It's arguable: according to the Anthrowhale,

Powerful build applies for grapple checks (giving a +4 bonus) but not for qualification to grapple (which is not an opposed check).

Obviously, the same thing could be said about the Jotunbrud feat...

And my favorite is that it arguably lets a small or smaller creature use swallow whole on a creature that’s a larger size category than it.

That's questionable - not just because of Grapple size limitations, but because you don't lose your feat-granted benefits while being magically reduced in size - would it be RAW for creature under the Minute Form effect to swallow something Tiny-, Small-, or even Medium-sized?
The similar, but closer to RAW case is - some monsters have Swallow Whole which lists exact sizes which it can swallow - such as Legendary Shark:

A legendary shark can swallow a Large or smaller creature