Here's another way to reproduce the error. The problem seems to be in the C implementation of _lru_cache_wrapper() / bounded_lru_cache_wrapper().

$ cat test.py
import functools
import threading
import time

@functools.lru_cache(maxsize=2)
def f(v):
    time.sleep(.01)

threads = [threading.Thread(target=f, args=(v,)) for v in [1,2,2,3,2]]
for t in threads:
    t.start()

$ ./python test.py
Exception in thread Thread-5:
Traceback (most recent call last):
  File "/somewhere/Lib/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/somewhere/Lib/threading.py", line 864, in run
    self._target(*self._args, **self._kwargs)
KeyError: (2,)

$ ./python
Python 3.7.0a0 (default:654ec6ed3225+, Dec 15 2016, 11:24:30) 
[GCC 4.8.4] on linux