This test relied on short-circuiting details of dictobject.py to avoid

crashing, and indirectly on the fact that hash codes in
random.randrange(1000000000) were very unlikely to exhibit collisions.
To see the problem, replace this number with 500 and observe the crash on
either del target[key] or del keys[i].

The fix prevents recursive mutation, just as in the key insertion case.

Lib/test/test_mutants.py

@@ -69,14 +69,12 @@ def maybe_mutate():
 
     elif keys:
         # Delete a key at random.
+        mutate = 0   # disable mutation until key deleted
         i = random.randrange(len(keys))
         key = keys[i]
         del target[key]
-        # CAUTION:  don't use keys.remove(key) here.  Or do <wink>.  The
-        # point is that .remove() would trigger more comparisons, and so
-        # also more calls to this routine.  We're mutating often enough
-        # without that.
         del keys[i]
+        mutate = 1
 
 # A horrid class that triggers random mutations of dict1 and dict2 when
 # instances are compared.