Long ago, someone was archiving magnetic tapes at MIT, containing Lisp Machine backups, from longer ago. Or maybe they were TOPS-20 backups... my memory has faded.
Archiving here means using an old 9-track tape machine with a custom driver, to copy data off now read-once 9 and 7 track tapes. Read-once tapes, because after the tape goes by the read head, the tape's plastic backing goes one way, and the tape's rust goes another. The backing is rewound, and the rust makes a scattered little pile. The original driver would backup and retry on error. Scrubbing back and forth, back and forth. Which here, would be bad. But back to our story.
On one such longer-ago backup, was a core dump file. A core dump, with a snapshot of the frame buffer. A frame buffer showing someone's screen, at that moment of the core dump, so long ago. And at that moment, that someone was being pranked. Pranked by a program which would draw, crawling across the screen, a little spider. A little bitmap sprite bug. A bug, trapped by chance in a core dump, and preserved in rust.