One of the Jackson 5's biggest hits -- "I Want You Back" -- was assassinated by former "American Idol" finalist Haley Reinhart ... who heinously botched the lyrics during a live performance, and TMZ has the footage.
You know Michael's famous words -- "oooh baby give me one more chance" ... but that's about all Reinhart remembered when she sang the Motown classic at the W Hotel in Hollywood last weekend.
In the video ... Reinhart forgets the lyrics in the VERY FIRST verse, momentarily gets back on track, and then flubs again -- all while peeking at the lyrics on her phone. Not that it helped.
Haley weathered a few hecklers -- like the dude yelling, "You don't know the words!"
Bright side? Her voice sounds great ... and her shirt is really shiny.