Didn't someone make a joke about Sesame Street comics once, and they're actually happening.
I think I did that. There's been a few other April fools threads I came up with that ended up becoming true in some way. The first April fools joke I did on the forum was regarding a collection of Sesame Street DVDs aimed at us. In 2005 somebody had contacted Sesame Workshop and was told that the company was considering releasing a DVD set aimed at adult collectors but it likely wouldn't be released until 2007. My post stated that they were able to clear various rights sooner than expected and that the DVDs would come out in 2006... And then we got the first Old School volume later that year.
I'm sure I've done a few other jokes that ended up happening, though I can't remember what off-hand.
Some people have mentioned the possibility of people not realizing that something is a joke. I would think that saying "April Fools!" at the end would work well enough. Though I have seen some (here and on other forums) where the users actually DIDN'T confirm it was an april fools joke in the first post. Some have, however, done something a bit more clever: Post some "news" and put random letters in bold, spelling April Fools.