Bike duplication glitch
The bike duplication glitch is a glitch in Super Smash Bros. Brawl that causes Wario to drop two of his bikes at once. While riding his bike, if Wario is hit so hard that he is knocked off of it on the same frame that he would normally fall off (due to the lack of speed and/or riding too long), the glitch will take place. Both bikes dropped are fully functional; they both "belong" to the originating Wario and so he cannot use the move again until both are destroyed.
As a glitch that only occurs on a specific and hard-to-predict frame, it is extremely difficult to intentionally perform. One method of attempting a consistent timing is to have the Wario player hold back and up as soon as the move begins, so that Wario turns around, pulls a very slow wheelie, and falls off after about 3/4 of a second. Because of the non-integer timings and positions, it's possible that the window available for the glitch happens in less than a frame, making it impossible to perform; slight variations in timing and positioning should be attempted.
When Wario starts the move, a prop bike is generated under Wario. This bike follows Wario's movement, and not the other way around. As a result, when the bike needs to be separated and converted into an item, the Wario object is responsible for disconnecting the bike at the correct time. Wario does this whenever he gets off the bike through any means. It appears that if he is hit on the same frame he gets off, both the "disconnect, I got hit" and "disconnect, I lost balance" segments of code are run, without one telling the other that it already disconnected the bike. As a result, both pieces of code create a bike item.