Jennifer Lopez's Skirt Falls Off Mid-Show, on Video

Jennifer Lopez might've been celebrating her birthday onstage ... but, it was fans who got the real present -- an eyeful of Lopez's backside when her skirt fell off!

The singer-songwriter took the stage in Poland Friday night ... and, while her fellow performers onstage sang a happy birthday song for her, Lopez's sparkling skirt slipped off her waist and ended up on the stage.

Check out the video ... J Lo's obviously not expecting the skirt to give way -- telling fans she's out onstage in her underwear before throwing the now-broken skirt into the crowd.

Jennifer says she's glad she's wearing underwear ... 'cause she doesn't usually -- so, fans almost saw a whole lot more!

Lopez's tour has been full of memorable moments ... like just last week when she posed in a series of sex positions in a steamy routine featuring a few of her backup dancers.

BTW ... Lopez turned 56 on Thursday -- and, she's still lighting up the stage and making fans pray for a wardrobe malfunction like this when they attend her shows.