I broke Atari Video Chess.

I found the glitch to rival Pokémon in Video Chess for the Atari Video Computer System (at least on the emulator AtGames made, Atari Flashback Blast! Vol. 2). If you flip the 1P difficulty switch to activate the game's built-in board-editing mode, then place three of your own kings on the board, one on a8, one on b8, and one on c8, then play a move, the game will basically implode. If it doesn't crash (which it does under most conditions), it will become corrupted. This glitch works because the king on a8 isn't in check, but the king on c8 IS in check. However, it only takes action if the first king it finds (in left-to-right, top-to-bottom order) is in check. Otherwise, it won't even tell you your king is in check or stop you from playing a move. I have found 4 moves that don't crash too often: placing a pawn on d4, removing the one on d2, and playing e4, playing d4, playing e4, or playing Nf3 and holding down Fire. The last one seems to not crash, guaranteed, as long as you hold down Fire. It has never crashed under that condition in my experience, except after executing its effects at least once. This isn't the only glitch that crashes the game, but it's the only one I've found that can survive. With stronger effects, it can also set your level mode to a glitched one, such as one that looks like a 6, inverted, with the last line of graphics replaced with a copy of the line above it. This one actually crashes if there's an enemy king on the board, or it isn't checkmate. It will still crash under this condition if there's more than one enemy king on the board, though this doesn't crash normally. There's a lot more to say about this glitch, and I'm only scratching the surface of what I discovered. I'm hoping someone else with an emulator (maybe don't try it on a cartridge) will be able to discover things I haven't. I know that there are 248 glitch level modes out there, each with their own speed and difficulty. I've found over 10 of them with this glitch. You can even stack corruptions for some crazy effects (unless one corruption overwrites the effects of another, such as sometimes a Nf3 corruption being overwritten by a d4 corruption)!

One thing to note is that this increases the chance that the game crashes, though.

Oh, and something I find a bit ironic: I once managed to have the game mode get set to Mode King (literally the graphics for the king). I'm presuming it plays like Level 7, the highest level in the game, but I didn't have the patience to wait TEN HOURS to find out. 

Hold down reset while turning power on with Space Invaders, and you will shoot two missiles at a time instead of one. Pew pew.

I guess I've been doing it all wrong - I usually try to avoid video game bugs...

Wow, I gotta say that's pretty lousy music to be hearing in your head. :-)

Oh, and another glitch that can be activated without using the editing mode: You can cause the game to move a "vBlank sprite" to the center of the screen, and cycles it between two spots. When switching game modes, it moves to the corner, and on the frame the mode switches, it returns to its normal position. I call it "Corrupted Double-Mate", even though it has no real corruption effects. It's actually because it reminds me of what I call a "Super Corruption" (the strongest corruption effects I can acheive with the glitch above. It mirrors the graphics on the c and d files to the g and h files. The g file has corrupted graphics which change based on what piece occupies a certain square, so it can be used to figure out what's on the g file, but the h file has no such thing). It's executed by placing your last piece on a8, which deletes it. However, this piece must be a queen or rook (pawns become queens before being deleted). Knights will activate another glitch, which I'll explain later in this post. How you would acheive this glitch is by winning the game against the computer (I suggest playing at Level 8, since it tends to blunder a lot), then taking all of the computer's remaining pieces while holding the checkmate (the computer is helpless to do anything since it's checkmate, and the game never prevents you from continuing to play after checkmate), taking the king (I don't know why this works), moving your dark-squared bishop (if you still have it) to a7, deleting it (don't know how that works, either, but placing it on b8 will crash the game if more than one move has passed since the last enemy piece was taken), moving your king to a7 (or a6 if a queen is on b8, otherwise, the game may crash depending on your position), deleting it also, promoting all pawns (they automatically become queens), lining everything up so that it can move to a8 on its next move (ignore pieces blocking the way), then moving a piece to a8. This turns the piece into an enemy piece (likely since the cursor is placed there by default as a failsafe of sorts when no enemy pieces remain), and it can be taken. Even if the piece can escape the checkmate, it's still checkmate for that move. Continue taking and the glitch will be activated. If the knight happens to be last, it creates the same "become an enemy piece" effect as before, you're not stalemated, though you have no moves, and you'll have to "hack in" another piece (with the board-editing mode) to continue or reset the board. A bishop creates the "Double-Mate" effect, where nothing glitchy really happens. Since both players have no moves, it defaults to checkmating you instead. If you're wondering, placing a king on a8 or b8 will crash the game if no enemy pieces exist, regardless of the circumstances, a knight on b8 will make an enemy queen appear on c6 (since the game has a rule where the last enemy piece becomes royal and cannot make any moves which would lead to it being taken, good luck handling that), a rook will make a bishop appear on d6 (cover your rook with another piece or it will be taken by the bishop), a pawn on b8 (possible only with the board-editing mode or the "Three Kings Corruption", as I call it, the glitch described in the first post) will make a knight appear on e6, and a "mated king" (an unused piece with no moves, only possible with some "Three Kings Corruptions") on b8 will make a rook appear on f6. Also, if you're curious, the opponent also has a "mated king" piece, although you cannot take it, so it's essentially a brick, since it has no moves unless it's on a8, in which the computer will spam Mc8 infinitely until it's given a piece by moving it to a8. The computer can take yours, though.

Believe me, the "Three Kings Corruption" can make much more ear-grating noises when executed, and they will last until they're overwritten by another sound. Be sure to keep the volume low when testing. Also, if you aren't expecting this effect to occur, it can make you jump out of your skin. The entire screen will display an extravaganza of glitched graphics, mostly consisting of lines running across the entire screen in one or two colors, and the glitched screens may update randomly in this glitched state. Depending on how it glitches out, this can be seizure-inducing.

Believe me, Caffeineed, this glitch (the "Three Kings Corruption") can create some of the most ear-grating noises known to man. I suggest keeping the volume low if you're going to try this. The game cycles through a bunch of glitched screens, mostly consisting of lines running across the entire screen from top to bottom, and in some scenarios, this happens very quickly and can be seizure-inducing. If you aren't expecting it to happen, it can make you jump out of your skin when it occurs.

Oh, the first one posted. The site "402'd" when that happened, so I assumed it didn't post.

I haven't found more glitches in the game, but maybe someone with an Atari emulator can find something I missed. I heard that sometimes, levels 6 and 7 will play two moves instead of one for whatever reason, but I've never encountered this in gameplay.

Actually, I forgot to mention that one of the ways the aforementioned glitch level mode (why it sticks out to me) can crash is this way:

I made a video detailing how to get this to work. For more details, I made another forum regarding this, which should be near this one (at least until edits and time separate them). It's labelled "Video Chess glitch showcase!". Anyways, here's the video (also on that forum):

https://drive.google.com/file/d/1k3rrQrHRP5N96B2zhQQnLL25o9IENDz9/view