Payday: The Heist, would be a prequel to Left 4 Dead. Valve's Chet Faliszek shot down that rumor, saying it was completely fabricated.
Faliszek stated, "I don't want to give it away, because we want the community to explore and find it, but I want to make sure that people don't think the prequel is coming."
However, he didn't rule out a possible prequel in the future, but added that the rumor might make an actual prequel less exciting than the fictional one.
"If that happens, then it will make this other thing we're doing feel uncool, when the thing we're doing is really cool. I mean, the problem is, if you make a rumour, and you can make the rumour anything, you're going to make the rumour really, really cool. And that makes everything else sound less cool," Faliszek noted.
Overkill did state two weeks ago that the game would be a "blend" of Left 4 Dead and Payday: The Heist. However, Faliszek stated that it would merely more like other crossovers in which two games were exchanging items much like Team Fortress 2 does with other games, where, say, players could earn items from Night at the Inventory for use in TF2.