Everybody knows the joke. Which is the "any key".

One would think that the only person to ever fall for this is Homer in The Simpsons - In episode 3F05 - "King Size Homer" - Homer's working at home because he counts as disabled (as he intentionally put on weight). The computer tells him to press any key and his reponse...

I see Esk, Catarl and Pig-Up. There doesn't seem to be any ANY key.

But no, perhaps someone else has asked this... as on the Compaq support website, you can find...

Compaq FAQ: Where do I find the "Any" key on my keyboard? (FAQ2859)
The term "any key" does not refer to a particular key on the keyboard. It simply means to strike any one of the keys on your keyboard or handheld screen.

However, this isn't really accurate. Because how many times have you seen "press any key", and pressed "Shift" or "Control" and nothing happens? Rumour has it that Compaq's more recent prompt is "Press Return to continue".