Similar to #16180 and possible a duplicate of #14743, if the script being run under pdb becomes invalid such as through a chdir operation, pdb will get trapped in a loop, catching its own exception.

$ mkdir foo
$ cat > foo/script.py
import os
os.chdir('foo')
$ python -m pdb foo/script.py
> /Users/jaraco/Dropbox/code/privacymate/harvester/foo/script.py(1)<module>()
-> import os
(Pdb) c
The program finished and will be restarted
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1667, in main
    pdb._runscript(mainpyfile)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1545, in _runscript
    with open(filename, "rb") as fp:
FileNotFoundError: [Errno 2] No such file or directory: 'foo/script.py'
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py(1545)_runscript()
-> with open(filename, "rb") as fp:
(Pdb) q
Post mortem debugger finished. The foo/script.py will be restarted
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1667, in main
    pdb._runscript(mainpyfile)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1545, in _runscript
    with open(filename, "rb") as fp:
FileNotFoundError: [Errno 2] No such file or directory: 'foo/script.py'
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py(1545)_runscript()
-> with open(filename, "rb") as fp:
(Pdb) q
Post mortem debugger finished. The foo/script.py will be restarted
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1667, in main
    pdb._runscript(mainpyfile)
  File "/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py", line 1545, in _runscript
    with open(filename, "rb") as fp:
FileNotFoundError: [Errno 2] No such file or directory: 'foo/script.py'
Uncaught exception. Entering post mortem debugging
Running 'cont' or 'step' will restart the program
> /Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/pdb.py(1545)_runscript()
-> with open(filename, "rb") as fp:
(Pdb)

I suggest PDB should always pass exceptions when the stack trace terminates in the pdb module.

Another way to reproduce this that will continue to work once #42383 is fixed is to delete or move the script while it's being debugged.