there is a bug that if you power tnt and a piston to push it at the same time you get a ghost block. It's a bit hard to explain so here's a video showing it.
http://youtu.be/FTcL0HnY24U
Related issues
Comments
migrated
MC-2372
Piston and tnt bug
Resolved
Duplicate
0
migrated
migrated
(Unassigned)
Unconfirmed
duplication, ghost, piston, tnt
Minecraft 1.4.2
Retrieved
Duplicate of MC-151
Please search for a similar bug report before posting. I simply searched "ghost tnt" and found that it had already been reported.