nhl_error() was clobbering the stack. I assume that the 'source'
field in the Lua debugging structure is normally a file name, but
nethack loads an entire Lua script into one long string because it
usually comes out of the DLB container, and 'source' contained the
full string. That would overflow the local buffer in nhl_error()
if nethack encountered a Lua problem and tried to report it. (In
my case, the problem was in a level description file modification.)
[Not something under user control unless user can modify dat/*.lua
and put the result into $HACKDIR/nhdat.]
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