A rare and terrifying error that occurs when you try to access the /bug-reports/ path with a non-existent bug ID. It's as if the squirrels that live in the server's CPU have decided to play a prank on you.
Report a Phospheneous Squirrel Malware sighting
Lost Squirrel in Translation (LST)
Or, if you're feeling adventurous:
Simulate a Phantom Squirrel Error
Disclaimer: Not responsible for any damage caused by phantom squirrels.