The Prometheus and Shame actor came along to "hang out" as a guest of Val Kilmer, who is cameoing in the BBC Two show's final episode.
Kilmer sent an email to producers asking whether he could bring a friend along to the set, and staff on the show were shocked when it was Fassbender who turned up.
A source told Digital Spy: "Fassbender and Kilmer were just hanging out with everyone on set and really enjoying themselves.
"It was amazing to see these Hollywood stars on the same set as the show's British TV stars like Les Dennis, Keith Chegwin and Shaun Williamson."
Life's Too Short cast in pictures - gallery:
Copyright: BBC PicturesOne person who wasn't shocked by Fassbender's arrival was the show's lead actor Warwick Davis, who confessed that he failed to recognise the actor.
"I didn't know it was him. He was just introduced to me as Michael and I just said, 'Alright Michael'," laughed Davis. "He probably just thinks I'm really cool now."
Filming for the Life's Too Short finale is currently taking place and it will air later this year on BBC Two.
Watch Warwick Davis talk about Life's Too Short below: