The best one that I heard of was the gag that PCformat pulled ia couple of years ago They setup a fake report that nintendo had produced a n64 card for the pc. The problem is that they hadn't heard of Ultrahle which had came out a few months earilier. So they got tons of emails from ppl correcting their fake story.
I actually have a 286 with a Sega Master system card inside. You have to move a small flap covering the floppy drive to make the Sega cartridge slot appear. I have not seen anything else like this. Anybody else know about this?