Joan Collins once mistook Rebel Wilson for a caterer.

The 80-year-old actress and the 'Pitch Perfect' star both guest starred on an episode of sitcom 'Rules of Engagement' last year and Rebel revealed Joan had no idea who she was.

She told ''[I once] did a television episode with Joan Collins. She thought I was one of the caterers, she didn't know I was an actress! She asked me, 'When's lunch?'''

Joan isn't the only star Rebel has had a run-in with, the 27-year-old actress was once told to ''f**k off'' by Russell Crowe.

Rebel studied comedy in New York after winning a scholarship funded by fellow Australian Nicole Kidman and when she spotted the actress having dinner with Russell in a restaurant, she thought it would be the perfect time to thank her.

However, Russell didn't even allow her to speak before telling her to ''f**k off''.

Rebel explained: ''I won a scholarship and I got to go to New York to study comedy and Nicole Kidman paid for everything.

''One time in Sydney I saw her having dinner with Russell Crowe and I thought 'Now's my chance', so I went up to them but before I say anything, Russell just turned to me and went 'f**k off'. So I just put my head down and walked away. That's a true story.''