Ariana Grande -- My Bizarre Obsessed Fan Won't Go Away!

Ariana Grande got a big scare at a concert after an obsessed fan with a bizarre history showed up, demanding a face-to-face with the singer.

Law enforcement sources tell TMZ, they were on alert for Timothy Normandin when Grande performed last month in Connecticut. There was good reason to have the heads up.

Normandin has had multiple run-ins with Ariana. He sent her various gifts, including a 42.5 pound pumpkin. He once showed up at her record company in a Santa suit. That stunt got him placed on a psych hold.

He was arrested at the concert venue for criminal trespass and released on a $5,000 bond.