Idina Menzel almost had a serious wardrobe malfunction onstage at Radio City Music Hall on Monday night.  But she didn't freak out.  

She just started cracking jokes about herself ... and then said a word that you'd never hear in "Frozen".  (She was wearing some leather bra-type thing under a dress that was open in the front, and the bra rode up so you could see some underboob.  

Someone from the audience pointed it out to her, and an assistant ran out and saved her.  Idina said, quote, "Well [eff] it, they're real."  

WARNING: the video has uncensored profanity.


She's cute ... but lest we forget the best Elsa impression HANDS DOWN ...