e-output-port) before its watch has been
removed from the event loop; if the watch is on a file descriptor and
the descriptor value is reused by the operating system, there may be a
phantom watch on the new descriptor.  Also note that, where a watch on
a port has just been removed, it is usually undesirable to close the
port in a different thread than the event loop thread because there is
no easy way of synchronizing the threads at that point.  If it is
necessary for a worker thread to close a port after r