A deadlock.

For example, the function is called immediately, rather than being run in a separate thread, causing it to block forever on accept(), because the connect() is after the call to async().

If concurrent() is used instead, the I/O implementation will spawn a new thread for the function, so that the accept() is handled by the new thread, or it will return error.ConcurrencyUnavailable.

async() is infallible. concurrent() is fallible.