When an exception reaches the top frame and is displayed by the exception handler, they are handled by formatting them to stderr.  This involves a series of calls to PyFile_WriteString, either directly from PyErr_Display or indirectly through PyTraceBack_Print.  Few, if any, of these PyFile_WriteString calls have their result checked for error.  Worse, PyFile_WriteString itself does not check the return value of fputs().

So, for example, with a signal handler installed, a signal received while a traceback is being printed can result in the traceback _not_ being printed.