NSYNC Star -- Sodomy Twist at Karaoke Night

Might sound crazy, but it ain't no lie ... one of the guys from *NSYNC changed the lyrics from "Bye Bye Bye" to include a line about anal sex ... and no, it wasn't Lance Bass.

It all went down Monday night at O'Shucks Karaoke Bar in Orlando, FL ... when Chris Kirkpatrick (the one who had the dreads) took the stage and busted out the N'SYNC classic with one of his buddies.

But when it came time to bust out the chorus ... Chris raised his middle finger to the crowd and joked, "But it ain't no lie ... baby f**k her ass."

The crowd laughed and continued to booze as Chris proceeded to rock out the rest of the song.