Square Enix has officially acknowledged the presence of a major bug in Kingdom Hearts 3D. The bug, while very specific, can prevent players from progressing further in the game.
Read Square Enix's information page for details on the exact nature of the bug and how to avoid the problem.
If you do encounter the bug, Square Enix recommends that you restart from your previous save file. If you do not have a save file, Square Enix asks that you contact their information center.