Last week's Morgan Freeman death hoax is only the broadest, most recent incidence of internet trolls hoaxing an easily transfixed Twitter audience. Tuesday afternoon some bored tweeter tried a little narrow-casting, making up a Matt Holliday injury, adding some odd specifics, and posting it to an @Cardinals_ account, for people who aren't looking very closely to notice and freak out about.
By 4 PM CDT it had spread through the #STLCards hashtag, the end result looking a little something like this after some civic-minded hashtag citizens Snopes'd it:
Chris Reed (@birdbrained) September 11, 2012
What causes one of these hoaxes to spread isn't necessarily the initial fake tweet, which I never even saw—most people might be able to tell something weird is going on, there—but the rapid responses to it that treat it as a fact and then, say, add on the tweeter's unpleasant hope that the Cardinals and their fans are ready to watch 20 games' worth of Shane Robinson in left field.
Because of the churn and conversation inherent to Twitter, this kind of thing is an easy hoax to spread—luckily, it's also an easy one to tamp down.