A few weeks ago, Payday: The Heist developers Overkill Software hinted strongly that a crossover was in the works for Payday and Valve's zombie shooter Left 4 Dead. Now it looks like a trailer for that crossover has been leaked and has made it on YouTube.
The trailer shows the Payday criminals inside the hospital that was the setting for No Mercy, the first campaign in Left 4 Dead. Kotaku got a confirmation from Valve's Chet Faliszek confirming this plan, adding, "There is a little cameo from one of the L4D characters (the comic should give you a hint) and a fun little easter egg with a payoff later in Left 4 Dead. Just us having some fun with a studio we are friends with and who have also made a great co-op game."
There were rumors that this crossover would also show the origins of the Left 4 Dead zombie outbreak but Faliszek denies this, stating, "It is not telling the origins of the Infection and is not canon."
This is likely to be some kind of future DLC download for Payday: The Heist, although there's no word yet on when it will be released or how much the DLC will cost. Overkill, which was recently acquired by Starbreeze, has said it is also working on a full sequel to Payday.