Thanks for the investigation.  I copied the limit from a python-ideas example and did not realize that IDLE adds so much to the call stack.  I should add a bit on the subject to the IDLE doc in "3.2. IDLE-console differences".  (It can also impact tracebacks -- as reported on a couple of tracker issues.)

"IDLE adds it own calls to the call stack before (and possibly after) running user code.  Tracebacks are filtered to remove these, but the filtering may not be perfect.  Setting a recursion limit that is too low, such as with sys.setrecursionlimit(20), will result in mysterious malfunctions."