Currently withMonitor is defined as

withMonitor :: ProcessId -> Process a -> Process a
withMonitor pid code = bracket (monitor pid) unmonitor (\_ -> code)

but it should be defined as

withMonitor :: ProcessId -> (MonitorRef -> Process a) -> Process a
withMonitor pid = bracket (monitor pid) unmonitor

Although it is possible, using the existing withMonitor, to wait for a MonitorNotification that matches the appropriate ProcessId, if another monitor exists for the same process ID then this may remove the wrong message from the process's mailbox.