Stone Half does not go away

[jdisk]

the best I can remember:

As a Stone warden, I had my two halfs summoned when I got dreamscaped.
When I returned, the Stone Half just did not go away. It doesn't move, it doesn't attack, but creatures take damage attacking it (from a lamp of scorching). Inspecting it, I get (amongst others)
HP unknown (die at -50)
Summon 0 turns.

If I walk into it, I get teleported some (21) squares away (always the same position). Creatures bumping into do not get moved.

I have the logfile saved if needed.