Andy Samberg, Akiva Schaffer and Jorma Taccone of comedy trio The Lonely Island were filming a movie scene in which an elderly woman's purse is stolen, when the 24 star was driving by the film shoot in Los Angeles.

He didn't see the cameras or recognise the actors and so jumped out of his car and rushed to the woman's aid, only to discover she was actually a stuntman in drag and the mugging was fake.

"I couldn't help but notice an elderly woman walking with a lot of groceries," he told U.S. late night host Seth Meyers on Thursday (18Feb16), "and the reason why it stuck out to me is because no one walks in Los Angeles and so I was kind of just watching her.

"Then out of the blue, behind a tree, jumps these two kids and they knock her down and they steal her purse. It p**sed me off, so I throw on the brake on the car and I jump out and I chase them down.

"I knock the one down and I grab the purse and I was just about to hit the other and someone is hitting me from behind and I turn around and it's the grandmother (mugging victim) with her cane, and I realise it's a guy."

Kiefer quickly realised he had stumbled onto a movie set, adding, "I look across the street and there's Andy with his camera crew of three people hiding in a tree and I'm like, 'Oh my God, I'm so sorry...' and just run and get back in my car.

"I thought that was the end of it, but the three guys that were making that film were incredible comedians and they showed it on (U.S. chat show host) David Letterman, like, five years later and I was embarrassed by that."