The Wiggler Glitch is a strange bug that occurs exclusively in Mario Kart Wii. It can be triggered when a piece of time trial ghost data is made that causes the racer to hit an object that they didn't hit during the creation of the run. The result is that the ghost data then becomes affected by an outside influence which causes it to desync, which then causes it to hit into walls or fly off the track even though the player never did that originally. Although this phenomenon is often referred to as the 'Wiggler Glitch', it can be triggered by many objects such as the bridge on DS Delfino Square. Ghosts subjected to this glitch are still legitimate, but can't play back correctly.
There are varying extents to which this glitch can corrupt ghost data. The most common result of ghost data corruption causes the ghost to drive around seemingly at random (although still trying to replicate the player's original inputs) in both replay mode, and when challenging it to a race. However, it is also possible to create a ghost that appears to play back perfectly in replay mode, as well as while racing from a distance, but that still desyncs when a challenger is keeping pace with it.
If a desynced ghost is allowed to play through to its completion, it will not shrink down and restart the replay from the beginning. Instead, the game will reach the end of its recorded inputs for the ghost data, and the character will simply sit motionless until the viewer chooses to end the replay manually. The reason for this is that the ghost is not triggered to shrink down and disappear until it crosses the finish line, and due to the desyncing process, few of these ghosts will ever get there.
What Can Be Done?
This is a bug in the game's code and therefore a Custom Track author or Time Trial racer cannot do anything to fix this bug. Sometimes the ghost will synchronize correctly using the live replay code, or using the "Challenge Ghost data" mode rather than the "Watch Replay" mode (or vice versa). This can therefore be used to prove a ghost's legitimacy despite the bug.
As of April 2016, MrBean35000vr is looking into fixing the original bug in the games code.