Rachel McAdams "hadn't gotten everything waxed" during a paddleboard yoga session.
The 34-year-old star was left red-faced when she was forced to work with a "handsome male instructor" while she enjoyed the sport with friends because she wasn't completely hair-free.
Quizzed on the last time she cringed, she told PEOPLE magazine: "Doing stand-up paddleboard yoga with some friends. There are elaborate poses, and we hadn't gotten everything waxed, and we had this handsome male instructor. I bought full-length board shorts for the next time!"
Rachel also admits that despite not having the best voice she opts to "belt" out lyrics to 'Is That All There Is?' - which was written by Jerry Leiber and Mike Stoller during the 1960s - as it special song which she shares with a pal.
She explained: "[The last time I sang was] in the car the other day, Peggy Lee's 'Is That All There Is?' I'm not a great singer, but that one is in my sweet spot. It's a song I share with one of my best friends in Canada, so I have to belt it. "