Some of these weren't necessarily repeatable, but:
I hit a glitch on my Taito Space Invaders: Part II upright once where I was on the far left side of the screen and fired a shot, and the shot or UFO or something must have wrapped around because it was destroyed on the right hand side, and I scored points for it, the remaining invaders quickly descended down the screen and my game was over (even though I had 2 lives left). Very weird!
It occurred during a 2 player game and my friend was able to continue their game
!
I also had a glitch occur while playing NBA Jam: Tournament Edition where my player jumped, and the background continuously scrolled up vertically for a bit with the player still in the foreground, then finally slowed down and came back to normal.
I believe Crazy Climber can do weird things if you get to view one of the large objects (e.g. King Kong), then climb back down so they're out of view and climb back up again. If I recall the large objects have the bottom and top halves inverted.
Some early versions of Galaga accidentally let you control the ship once it gets beamed up in the demo mode. For some reason they used this version in the Class of '81 machines and you can cause it to crash/reboot by taking control and shooting enemies
My old Exidy Venture machine had an unreliable power supply and would be pretty fun to play when the voltage would drop. It would cause holes to appear in the maze walls that you could walk through and wrap around to the other side.
