On a try/except-block if an exception raises (for example KeyboardInterrupt) the except block could cause another exception and if this block tries to catch it too the nested except block could cause another exception again. This goes into an unlimited recursion.

In the attachments is an example of such a problem (race_condition_fast.py). But as it is called a "race condition" it is nearly impossible to reproduce it by a human. For this case I have adjusted the example (race_condition_slow.py). The third CTRL + C will cause a KeyboardInterrupt.