Depends on the game honestly. My favorites have been from the Zelda series, aside from the two game breaking ones in the past two console games. Doing stuff like the fourth day glitch in Majora's Mask, the entirety of keeping the wooden sword for good in Twilight Princess, etc., have been very interesting as well as an unintended challenge. It's almost like looking behind the scenes, figuring out how exactly the world is programmed.
And then you have games like Sonic '06 which throw glitches at you all the time. If it's not a terrible story, painful voice acting, or bad game design that makes you stop playing, it'll be falling through the floor or Sonic losing all speed on a loopdeeloop and falling to his death that will have you snapping the disc. Games like Fallout have similar game breaking bugs but at least everything else redeems them.
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