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The weirdest thing that happened to me was I was surfing in the pool of water in the back of the day care center and got attacked by a tentacruel. I don't have a whole lot of glitches in my games.
I have witnessed 2 main glitches and had one of my own
1: I was playing emerald on VBA and put in a code to get a suicune in my PC but it was a bad egg
2: My little bro was playing his (real) sapphire and he was talking to someone in the battle tower, then he watched TV for a while when he looked at his game he was in Lilycove city and to top that there was grass in the city! he searched the grass and found a Misingno!
3: my best friend bred 2 charazards on diamond he got an egg and from it hatched a shiny charmander with fly!
My Yellow's Bulbasaur being cloned when the battery ran out while trading with Silver. I didn't know about cloning back then, so it freaked me out. Lucky only some fodder Caterpie or something was wasted, instead of my only Bulbasaur.