GHC.Event overwrites main IO managers hooks to RTS
The IO manager registers two file descriptors with the RTS which the RTS uses to send control and wakeup signals to the IO manager. The main IO manager is started up by default and registers some file descriptors that it has allocated with the RTS.
The base package also exposes a GHC.Event module which when initialized will also register files with the RTS, overwriting the main IO manager's files. Now the RTS can no longer signal the main IO manager.