To reproduce:

```
import threading
import time


class U(threading.Thread):
    def run(self):
        time.sleep(1)
        if not x.ident:
            x.start()


x = U()
for u in [U() for i in range(10000)]: u.start()

time.sleep(10)
```

Chance to reproduce ~20% in my setup.
This script has a data race (check then act on x.ident).
I expected it to occasionally hit `RuntimeError: threads can only be started once`

Instead, I get:
```
Unhandled exception in thread started by <bound method Thread._bootstrap of <U(Thread-1, started 139798116361984)>>
Traceback (most recent call last):
  File "/usr/lib64/python3.5/threading.py", line 882, in _bootstrap
    self._bootstrap_inner()
  File "/usr/lib64/python3.5/threading.py", line 906, in _bootstrap_inner
    del _limbo[self]
KeyError: <U(Thread-1, started 139798116361984)>
```