— haha yeah man (@JayFromTGF) December 7, 2017
The 22-year-old man from Wolverhampton put his head in a plastic bag inside the microwave before friends poured seven bags of Polyfilla inside.
The group had intended to use the microwave as a mould, but the man got trapped and by the time emergency services arrived, the group had spent 90 minutes trying to rescue him.
West Midlands Fire and Rescue said they were "seriously unimpressed" with the group.
"As funny as this sounds, this young man could quite easily have suffocated or have been seriously injured," Watch Commander Shaun Dakin was quoted as saying by the Sky News.
"Taking the microwave apart was tricky because a lot of it was welded. We video-called our technical rescue colleagues for advice and eventually managed to get him unstuck," Dakin said.
The man was able to breathe with the help of an air tube the group had managed to feed through.
"He was very relieved when we removed a large chunk of the Polyfilla with a screwdriver, allowing him to breathe more easily. But we had to be extremely careful with the screwdriver, working so close to his head," the official said.
"It took us nearly an hour to free him. All of the group involved were very apologetic, but this was clearly a call-out which might have prevented us from helping someone else in genuine, accidental need," he added.