The singer was doing an homage to Tina Sunday night during the broadcast ... undoubtedly a huge moment in the show considering TT passed away last month. Patti was a natural choice to sing her songs -- but in the middle of her performance, she stumbled.

Check it out ... while PLB was cranking out Tina's classic hit "The Best," she paused for a bit and said she couldn't see the words. It got a laugh from the crowd, and Patti carried on as best as she could. However, her confession definitely spurred a lot of head scratches.

On its face, it looked like Patti was reading the lyrics somewhere off in the distance ... and hadn't actually memorized them. That was later confirmed to be true by BET itself.

A rep for the org says, "It was an extraordinary privilege to celebrate the life and legacy of Ms. Tina Turner at BET Awards 2023. Having the incomparable Ms. Patti LaBelle on our stage honoring one of her contemporaries was an equal privilege."

They tried covering for her, adding ... "Due to the crowd's enthusiasm, the teleprompter was obstructed, obscuring Ms. LaBelle’s view of the lyrics. Nonetheless, we couldn’t be more grateful to Ms. LaBelle for lending her incredible talent to this moment."

Like we said, no true harm or foul -- and it ended up being a funny moment big picture. But, truth be told, some were kinda disappointed in Patti for not being better prepared ... especially in a big tribute to another musical great on such a grand stage.

BTW, this ain't the first high-profile mishap Patti's suffered ... remember back in the '90s when she was singing for the Clinton White House, and her whole "backup singers" fiasco??? Yeah, that one wasn't really her fault ... but this sure does have echoes of that.