Situations where you look at the rule(s) and think to yourself
"Man, this is ridiculously stupid! There's no way it works like that in my game!". Sometimes it's nothing major, like the fact that a Monk isn't proficient in unarmed strikes. You can houserule it and then it makes sense to you and everyone is happy.
But sometimes, when you houserule (or interpret) it your way you will make life harder for classes or concepts that already have it hard. What then? Leave it and be annoyed by it or change it, but shaft some players character?
To the point:
Did you have such a tricky situation? What was it?
Would you change it or leave it?
Do you know of any rules that could lead to such tricky situation?
One example: Dragonwrought kobolds.
(although it's more about balance and cheese)