Pet peeve for me is when you rant to your friends about how people want something really unreasonable for a legendary or a shiny and their response is " why not just hack it into your game and call it a day?" or "just hack it"
Kids wanting to battle you with a hack team of shiny legendaries with 9999 hp *happened more frequently on older games*
Willing to make a trade with someone but cant tell me honestly if its hacked/legit or even if it was untrained or mixed ev'ed.
NPC Trainers that spot you when your were certain you sneaked behind him/her or walk into 3 npc trainers looking about and after you defeat one another spots you before you can move or recover.
people d/cing after a victory in wi-fi battles.