Flavor Flav to Miley Cyrus -- Nice to Meet You ... GWEN STEFANI

And this week's painfully cringeworthy award goes to ... FLAVOR FLAV ... who met Miley Cyrus backstage at the iHeartRadio Music Festival and was convinced she was GWEN STEFANI.

The video was shot Friday night in Vegas -- showing Flav bumping into Miley backstage and getting really excited ... blurting out, "GWEN STEFANI!!!"

Miley doesn't correct him, she just smiles and gives him a hug. Then Flavor calls her Gwen again.

At that point, someone in Flav's camp pulls him aside and tells him it's actually Miley Cyrus.

At first, Flavor can't believe it -- but after the penny drops, Flavor tries to make amends ... turning to Miley and saying, "Do me a favor. Smack me."

Funny ... Gwen was also there Friday night ... looking NOTHING like Miley.