The Cheers alum, 53, started off his opening monologue noting that the first time he hosted the sketch comedy show it was 1989. Naturally, he quickly brought up Taylor Swift's hugely successful new album named after that year.
Picking up a guitar, he said, "With all due respect to Miss Swift, I think I know a little bit more about 1989 than she does." He proceeded to sing his own funny lyrics to the tune of the Grammy winner's new single "Blank Space."
Then Harrelson's costars Hemsworth, 24, and Hutcherson, 22, came on stage to help out, explaining they weren't even born in 1989. Moments later the audience screamed as Lawrence, 24, came on stage. Harrelson quickly dropped to the ground, hugging her leg, and exclaiming, "This is amazing! The real Taylor Swift is here!"
"Woody, I'm not Taylor Swift, I'm Jennifer, we've done about 10 Hunger Games movies together," the Oscar winner said.
Up next Harrelson suggested they all sing along. Lawrence, who joked on The Late Show With David Letterman about her terrible singing voice earlier in the week, noted, "When I sing I sound like a deer that's been caught in a fence."
"I would say it's more like a dog that's been hit by a truck," Hemsworth quipped.
"Or like one of those goats that screams like a human," Hutcherson added.
Watch the clip to see what happens when Jennifer Lawrence is forced to sing and her adorable flub right before that moment!