If you subclass OSError without calling OSError.__init__() then you can get a crash.  For example

  Python 3.3.0b1 (default:cfbe51e66749, Jun 30 2012, 20:50:54) [MSC v.1600 32 bit
  (Intel)] on win32
  Type "help", "copyright", "credits" or "license" for more information.
  >>> class MyError(OSError):
  ...   def __init__(self):
  ...     pass
  ...
  [61116 refs]
  >>> repr(MyError())
  Assertion failed: obj, file ..\Objects\unicodeobject.c, line 2646

Note that str(MyError()) results in a crash without hitting a failed assertion.

The problem does not occur in Python 3.2, and it does not affect subclasses of Exception.

Here is a quick patch (needs a test):

diff --git a/Objects/exceptions.c b/Objects/exceptions.c
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -834,6 +834,7 @@ oserror_init(PyOSErrorObject *self, PyOb
 #endif
 
     /* Steals the reference to args */
+    Py_CLEAR(self->args);
     self->args = args;
     args = NULL;
 
@@ -916,6 +917,11 @@ OSError_new(PyTypeObject *type, PyObject
             ))
             goto error;
     }
+    else {
+        self->args = PyTuple_New(0);
+        if (self->args == NULL)
+            goto error;
+    }
 
     return (PyObject *) self;

New changeset 1cbab581bf1e by Antoine Pitrou in branch 'default':
Issue #15229: An OSError subclass whose __init__ doesn't call back
http://hg.python.org/cpython/rev/1cbab581bf1e

It should be fixed now.