Not a joke because it is intentional but I always found ssh's answer to an non-existent user hilarious:

    ssh user@host
    You don't exist, go away!

Raw material for further jokes:

    head -c4 AnyJava.class | xxd -p
    cafebabe

The more morbidly inclined AIX user might try this with tail on a coredump and will find deadbeef.