Consider this program:

import pdb

pdb.set_trace()

print ("At line 5")
print ("At line 6")
print ("At line 7")
print ("At line 8")
print ("At line 9")

When set_trace starts the debugger, I set a breakpoint at line 8. When I
do that, the continue command starts single-stepping instead of what it
usually does.
Also, the module will appear twice on the call stack (although pdb won't
show this).

Here is the pdb session:

$ python pdbfail.py
> /home/petr/tmp/pdbfail.py(5)<module>()
-> print ("At line 5")
(Pdb) break 8
Breakpoint 1 at /home/petr/tmp/pdbfail.py:8
(Pdb) continue
At line 5
> /home/petr/tmp/pdbfail.py(6)<module>()
-> print ("At line 6")
(Pdb) continue
At line 6
> /home/petr/tmp/pdbfail.py(7)<module>()
-> print ("At line 7")
(Pdb) where
> /home/petr/tmp/pdbfail.py(7)<module>()
-> print ("At line 7")
(Pdb) quit
Traceback (most recent call last):
  File "pdbfail.py", line 7, in <module>
    print ("At line 7")
  File "pdbfail.py", line 7, in <module>
    print ("At line 7")
  File "/usr/lib/python2.5/bdb.py", line 48, in trace_dispatch
    return self.dispatch_line(frame)
  File "/usr/lib/python2.5/bdb.py", line 67, in dispatch_line
    if self.quitting: raise BdbQuit
bdb.BdbQuit