Chevy Chase Falls Off Stage At 'Christmas Vacation' Movie Q&A

Chevy Chase is falling hard for the holidays, but not in a good way -- he tumbled off a stage during a 'Christmas Vacation' event, and despite Clark Griswold's smartwatch having some serious concerns ... he's doing fine.

Chevy was greeting fans Wednesday night at Shea's Performing Arts Center in Buffalo, NY ... where they screened his National Lampoon holiday classic, but as you can see in this clip we got, the Q&A sesh got off to a rocky start.

For some reason, Chevy was rolled out onstage in a wheelchair, but quickly stood up and started walking toward the crowd ... before suddenly dipping out of sight, as he stepped right off the stage!

He eventually got up and back to his seat onstage with help from his wife, Jayni, and the Q&A's host ... and the whole room had a laugh when Chevy's smartwatch called him out for his tumble!

Folks in the audience told us he ended up doing the event with an icepack on his knee but still stuck around to snap some pics with fans and answer questions.

Sources close to Chevy tell us he simply misjudged the edge of the stage when walking out -- saying the stage lights on him blacked it out ... but he's doing just fine, apart from a bruised knee.