Stephen Mulhern cried out in exasperation on Saturday night's episode of Catchphrase after the final two contestants kept insisting a door knocker was a bell.
In the popular ITV game show, players are asked to "say what you see" in order to have a chance of winning £50,000 cash.
Finalists Hannah and eventual winner Sinead were both stumped by an animation with children in a school. The two women suggested phrases containing the word bell despite being told on more than one occasion that it was not a bell.
"That's not a bell," said Stephen.
After hearing "ring the school bell" - he lost his cool.
"That's not a bell!" he loudly exclaimed. He then walked over to the board and said again: "It's not a bell!"
In the end, Sinead was correct with "school of hard knocks". She also ended up winning £14,100 plus a holiday to New York.
Viewers who had their say on Twitter enjoyed seeing the normally unflappable Stephen jokily lose his cool.