It's a very common bug that ha been around ever since the game started. It has gotten a lot less common but still happens from time to time. It will soon fix itself. Anyway, it's a bug so I've moved it to the bugs section.
For future reference, if it walks like a bug, talks like a bug, smells like a bug or just plain looks like a bug.....it's most likely a bug. So post it in the bugs forum.

As soon as you build an idiot proof system, somebody else builds a better idiot.
If it moves, kill it. If it doesn't move, kick it until it does move, and then kill it.