The code in getTerrainChar() isn't doing anything at all to Pygame.  I
can't get a reliable repeat of this--it happens at different places, so
far not in my graphics code itself.  It's always taken 5 or 10 minutes
into play.  There's only one thread, though there is a Clock.
  Any ideas?
this is almost always a case of some sort of reference count bug. this 
can only happen in the "C" python modules. it probably is a pygame 
bug, unless you suspect it is some other C module you are using? what 
version of pygame are you on? something in your game must be 
triggering a bug. if it is a pygame thing it's nothing that i've 
noticed yet.