The classic UNIX Magic poster by Guy Overacre was distributed at USENIX conferences.
It features a white-bearded wizard with UNIX-related things around him — a spool of thread, a black cat, a boot, a fork, pipes, buckets, a number of containers — labelled with things like tar, null, awk, uucp. There is even a 'C' container, and a partially obscured and broken 'B' container.
The wizard is pouring mysterious liquids into a giant shell.