Good Bad Bugs/Shoot Em Ups


 * Sinistar had a bug that allowed you to acquire 255 ships. It takes a second or two for the Sinistar to eat your ship, during which a Warrior ship can shoot you, making you lose two lives at once. If you started with one life, your number of lives would drop from 1 to 0 to -1. Since this number was stored as an 8-bit unsigned integer, -1 would register as 255.
 * Later games in the Touhou series have Marisa's B firing mode glitched up. In Mountain of Faith and Subterranean Animism, using Marisa B with 3-3.95 Power caused insane damage, enough to cut lengthy cards to a third of the length. It's a bit nerfed in SA, though, as you need to shotgun with a specific formation. It also fails in MoF if the player focuses.
 * And akin to this, "Malice Cannon" in Imperishable Night. As the Magician Team (Marisa and Alice), repeatedly tapping the focus button allows both characters' attacks to be used simultaneously. This bug doesn't break the game, but rather moves the ridiculously underpowered Marisa/Alice team closer to the damage output of the other teams.
 * The Malice Cannon actually buffs up the Magic Team (Alice/Marisa) to the strongest team in the game, with a ridiculous damage output, although the player still has to deal with the fact that it only fires in front of them. Technically, all four teams have the ability to perform this bug, however, the Malice Cannon is the most notable.
 * Older Than the NES. In the Combat game packed in with the original Atari 2600, running your tank into the inside of a corner--usually a corner of the screen--would teleport you to the opposite side.
 * In Tank Pong, after one player successfully kills the other and his tank is still in motion during the "death sequence" (i.e. the "dead" opponent spinning), if the victor's moving tank touches the wall bounding the playfield, his tank will suddenly start sliding rapidly along the wall, warping around off the edge of the screen and back out the opposite edge, until the next round begins. If the same victor's tank touches the beaten tank while doing so, he also takes the spinning tank with him in that joyride.
 * In any Tank game, having both players' tanks positioned one right in front of the other (i.e. one Tank's gun right on the other's rear) and touching, and having both move forward, will cause both to suddenly begin spinning rapidly in a circle (and passing through any nearby wall or barrier).
 * In Afterburner Climax, after lighting the afterburners for a while they would automatically turn off and your plane would slow down to cruising speed. By deliberately throttling back from the standard middle position and then shoving the throttle forward again periodically, you could maintain afterburner near-continuously. This was not removed from the XBLA and PSN releases.
 * In the original Escape Velocity, one could suppress planetary defense fleets entirely by hiring enough escorts to achieve the Cap on the number of ships in a system.