Good thing I checked for existing topics before posting... also, good thing I only spent 3 minutes on my example hold.
I can confirm this exists on build 54, and that it works if anything breaks the wall (such as a sword).
Meh, maybe I will post the hold anyways, because I just found another glitchy graphical situation involved with Build and added it in - some of the cliff image is retained when you replace a Floor surrounded by pits with a Bridge/Platform/Whatever.
Pardon the hold's name.
Also, my DROD.err is a little different. I have about a hundred of this:
Assertion error in line 7676 of .\DbRooms.cpp: "bIsPlainFloor(this->coveredOSquares.GetAt(wX, wY))"
And only one of this, sitting somewhere just randomly in the middle:
Assertion error in line 4567 of .\DbRooms.cpp: "bIsPlainFloor(this->coveredOSquares.GetAt(wX, wY))"
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Also known as ExpHP everywhere else.