There is a race condition in create_connection() used with wait_for() to have a 
timeout. sock_connect() registers the file descriptor of the socket to be 
notified of write event (if connect() raises BlockingIOError). When 
create_connection() is cancelled with a TimeoutError, sock_connect() coroutine 
gets the exception, but it doesn't unregister the file descriptor for write 
event. create_connection() gets the TimeoutError and closes the socket.

If you call again create_connection(), the new socket will likely gets the same 
file descriptor, which is still registered in the selector. When sock_connect() 
calls add_writer(), it tries to modify the entry instead of creating a new one.

This issue was originally reported in the Trollius project, but the bug comes 
from Tulip in fact (Trollius is based on Tulip):
https://bitbucket.org/enovance/trollius/issue/15/after-timeouterror-on-wait_for