I recall one particularly cruel April Fool's joke from Mattel.
Some of us at He-Man.org had been clamoring for some time for them to FINALLY produce a "Fearless Photog" figure. If you're unfamiliar with "Fearless Photog", here is a Squidoo article I created some years back regarding him (the main source, www.fearlessphotog.com, is no longer around).
Anyway, one year Mattel announced they would be premiering the figure as part of their "Masters of the Universe Classics" line of collector toys.
This was, of course, an April Fool's joke. It was very obvious from the start. But it still rubbed salt on the wounds of those who wished Mattel would finally live up to what they had promised nearly 30 years prior.
Well, they did actually release a "Fearless Photog" figure a couple years later ( I own 7 of them still in the box, and 1 loose ).
They even took the time to contact Nathan Bitner (his creator) and interview him regarding the release of his creation at long last. So, in the end, they knew they would be satisfying those of us who desired the figure... but thought it would be funny to tease us first.
This is why I hate April Fool's Day now.