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The following example works on Python 2.7 and 3.2, but fails on Python 3.3:
-----------
import errno
import os
try:
 os.rmdir("testdir")
except:
 pass
os.mkdir("testdir")
try:
 try:
 os.mkdir("testdir")
 except IOError as exc:
 # If can't get proper access, then just forget about writing
 # the data.
 if exc.errno == errno.EACCES:
 pass
 else:
 raise
 except OSError as exc:
 # Probably another Python process already created the dir.
 if exc.errno == errno.EEXIST:
 pass
 else:
 raise
except Exception:
 print("PEP 3151 broke backward compatibility on such pattern!")
-----------
I noticed the problem while reading the changeset e4d44c2e8e81.

Why would you catch IOError after os.mkdir()?

The first example was extracted from Lib/importlib/_bootstrap.py. The code was maybe wrong, I don't know.
Another example:
----------------------
import errno
import os
try:
 os.rmdir("testdir")
except:
 pass
os.mkdir("testdir")
try:
 try:
 #os.mkdir("testdir")
 open("NOT EXISTING FILENAME")
 except OSError as exc:
 if exc.errno == errno.EEXIST:
 pass
 else:
 raise
 except IOError as exc:
 if exc.errno == errno.ENOENT:
 pass
 else:
 raise
except Exception:
 raise
 print("PEP 3151 broke backward compatibility on such pattern!")
----------------------
Uncomment mkdir() to test both paths.

The code was fixed in importlib. I don't think that this borderline case should be documented anywhere, so I close this issue.