We've seen tracebacks in production like:

  File "<frozen importlib._bootstrap>", line 1004, in _find_and_load(name='oe.gpg_sign', import_=<built-in function __import__>)
  File "<frozen importlib._bootstrap>", line 158, in _ModuleLockManager.__enter__()
  File "<frozen importlib._bootstrap>", line 110, in _ModuleLock.acquire()
 KeyError: 139622474778432

and

  File "<frozen importlib._bootstrap>", line 1004, in _find_and_load(name='oe.path', import_=<built-in function __import__>)
  File "<frozen importlib._bootstrap>", line 158, in _ModuleLockManager.__enter__()
  File "<frozen importlib._bootstrap>", line 110, in _ModuleLock.acquire()
 KeyError: 140438942700992

I've attached a reproduction script which shows that if an import XXX is in progress and waiting at the wrong point when an interrupt arrives (in this case a signal) and triggers it's own import YYY, _blocking_on[tid] in importlib/_bootstrap.py gets overwritten and lost, triggering the traceback we see above upon exit from the second import.

I'm using a signal handler here as the interrupt, I don't know what our production source is as yet but this reproducer proves it is possible.