When I wrote this plugin I thought nail grens which are stuck in a wall also trigger this bug, but apparently that's not the case. So it needs to be modified to only remove nail grens which are inside a ceiling.
I've reverted to Sparky since I discovered this, however I have problems with the server sometimes crashing with a "corrupted double-linked list" error which I suspect Sparky is causing it. So this is another reason to use the AMXX plugin instead of Sparky, just needs to be tweaked.