Well, this is odd. We've known about an upcoming Payday: The Heist-Left 4 Dead crossover for about a month now, and - as is often the case with these things - it materialized earlier today via YouTube's eyebeaming ethers. ""Have you ever wondered how the Left 4 Dead series began?" the video's description asked, forcing me to realize that I've never actually wondered that. "It started with a heist!" Intrigued, the Internet looked on as Payday's band of mask-loving hooligans shot its way through a very un-zombified version of Left 4 Dead's Mercy Hospital. And then the trailer disappeared.
Happily, Eurogamer managed to upload a roll to its own Internet-powered projector machine, so you can still watch the trailer and remember it as it was before its mysterious disappearance. Perhaps Valve and Overkill (now known collectively as Valverkill) stole it. From themselves. This may just be a heist caper for the ages. I, however, will work tirelessly to get to the bottom of it: Oh look, here's the answer. It's just a Left 4 Dead cameo (character and hospital setting) in a Payday map.
Hooray! Sort of, eh?