BEN AFFLECK hosted an at-home viewing party for producers and studio execs to see his upcoming flick “Runner, Runner,” but his VIP guests gasped when he fired up his DVD player and… SURPRISE!…a private “home movie” flashed up on the screen!

Turns out it was…(don’t get ahead of me here, folks)…NOT  a sex tape starring Ben and hot wife JENNIFER GARNER, but scenes of Ben in a Santa suit singing Christmas carols to Jen, kids and friends at an Affleck Yuletide party.

Quickly changing discs as the studio bigs chortled, Ben said: “Jen and I have been looking everywhere for this!”

He’d had the party scenes edited onto a single disc to make copies for pals, then lost track of it.

Incredibly, when he’d reached into his disc stack to retrieve “Runner, Runner,” he unwittingly glommed onto his missing DVD!