A very popular glitch was to hold the reset button while turning the Atari 2600 console with Space Invaders in it. It gave your single-player tank a double-shot. I'm no longer convinced it was unintentional.
Mountain King (originally by Imagic)
There was an accidental "super jump" in the game. If you timed a jump just right, by releasing the stick for a split-second and then hit "up" again, the man would make a half-jump followed by a jump that would fly up into the air until it hit something. This caused the game to move into ROM and RAM addresses creating a "sky world" to explore.
On Leaderboards in general, "rollover glitches abound." Mario Kart on the Wii had rollover glitches to the timer, making them patch the game and reset the leaderboards. I noticed the more recent "Cuboid" on PSN has the same problem! Both the number of moves and the timer rollover to zero making it possible that somebody willing to make 1000 moves and wait 99:99 minutes (or whatever the maxes are) to get the top leaderboard position.