The Kecleon shop where you pick up an apple and accidently leave with it. Then all these random high leveled Kecleon come and SMASH YOU!
Probably escort missions that require you to escort a lv 1 pokemon through a lv 40+ dungeon. They always seem to die somehow and you ALWAYS have to make sure no enemy pokemon is close to them.
i especially hate when u are so close to the end of a long dungeon and you die, leaving all the really good items and cash you brought behind. AARRGGHH!
(i also hate monster houses.)