Unintentionally hilarious glitches?
Oh dear God where to start?!
I've experienced far too many to even begin to recount them. There's just....so many. However, one of my favorites is from back in the Nintendo 64 days.
There was a bug in a game whose name I'm completely blanking on at the moment, that was purposefully left in the game during and after testing. (as in, it occurred unintentionally during the design phase but was left in later)
Basically, what it involved was the following:
At the cheat menu, you entered in a certain sequence of characters. I think there was more than one code needed, but again...my memory is failing me at the moment.
After you entered in the code(s), the glitch got weird. Basically, you would grab the cartridge, while it was still plugged into the console, and slowly lift the left side until the game crashed. Then, you pushed it back down and pressed the rest button.
Upon doing this, the game would boot up into a developer debug menu system. You couldn't do much within this menu using the controller, but it was still a rather interesting glitch that was both unintentionally created by the devs and later used by those same people for debugging purposes.