When lastseentyp[x][y] is different from levl[x][y].typ and #terrain
tries to show <x,y>, the value of levl[x][y].flags might not have an
approrpriate value for the remembered terrain type. The reported
problem was an impossible() about wall mode.
rm.wall_info == 4 corresponds to rm.doormask == D_OPEN and both of
them are overloaded on rm.flags. A spot remembered as a wall but
actually a secret door might cause this if it has become discovered
('wall' changed to closed door) and then opened (with door intact)
while out of view. [I'm not sure how that could happen though.]
I was unable to reproduce it so haven't verified that the fix works.
14e35ce419