When a Pokémon you really want to catch appears, and you use your weakest attack to whittle it down. Guess what? CRITICAL HIT HURRDURR!
When you're trying to catch a legendary, and the PokéBall wiggles 3 times and you start celebrating only to hear the stupid thing break out. Ugh.
When gym leaders or anyone otherwise uses a potion when you're about to knock out their Pokémon.
Battle Subway. Don't even get me started on the hax in that place.