The developers of Payday: The Heist (Overkill) have teamed up with Valve to create a DLC for Payday that's set within Left 4 Dead's Mercy Hospital map in the No Mercy campaign.
The objective in each Payday: The Heist mission is to rob the place and then fend off against the police response as the lights go out.
Speaking on the No Mercy map, Valve's Chet Faliszek said that the mission is not canon to the Left 4 Dead story. However, he added that there is a small cameo from one of the characters anda "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."
The No Mercy DLC for Payday: The Heist will be released soon.