Each song, on Thursday, was translated into Lithuanian and then back to English, resulting in completely different lyrics.
Cabello was up first and performed Billie Eilish's "Bad Guy" which had turned into "Evil Young Boy."
"An evil young boy/ That sounds like a bumpy boy/ Pude insults your husband/ Sternum's under pressure/ My text is evil/ Your mom gets kind of weird/ My daughter's a desert/ Eliminate your dad," Cabello sang, slightly mimicking Eilish.
Fallon then performed "All Star" by Smash Mouth which became "Hot Ball" before the pair teamed up for a duet of Shaggy's "It Wasn't Me," which turned into "I'm Not The Self."
"I was spotted on a mantle/ I'm not the self/ I was harmful to the couch/ I'm not the self/ Bathing very much with her now/ I'm not the self/ She took a picture of receptions/ I'm not the self," the duo sang with both Cabello and Fallon impersonating Shaggy.
The pop star also discussed on The Tonight Show her relationship with fellow singer Shawn Mendes and how things were a little weird at first since she was friends with Mendes for a long time beforehand.
"It took me two months to be able to like call him baby," she said. "I didn't want to say his name because we were dating and I couldn't call him baby so I'd just be like 'Hey, you!'"
Cabello's second solo album titled Romance was released on Friday. The project contains the songs "Senorita" with Shawn Mendes, "Liar" and "Living Proof" which the 22-year-old performed on the Tonight Show stage.