If you didn't start your adventure yet there's a fun glitch that breaks the game. Then when you actually start the game the mew glitch is fun. Missingno is fun but it's gamebreaking. And there's also glitch city. This is a great video of the first glitch I mentioned cause I'm not sure how to describe it:
https://www.youtube.com/watch?v=OyhEKG_g53oIf you don't teleport you'll notice that you have about a million pokemon in your party, when I was messing around with this I noticed that when I switched some of the pokemon around I got glitched clones of certain pokemon. I recommend trying it cause it's very fun.
