Quiz channel ITV Play broke broadcasting rules by making its answers too obscure.
ITV Play has earned millions for its parent company
Viewers complained after two answers to the question "what items might be found in a woman's handbag" were revealed to be balaclava and Rawl plugs.
Ofcom ruled the answers were unreasonable and the competition was therefore not conducted fairly.
ITV Play apologised for what it described as an "exceptional one-off error of editorial judgment".
The channel, which encourages viewers to take part in games via premium rate phone lines, has faced criticism for giving viewers little chance of getting through to a programme.
But ITV Play has insisted it strives to produce popular and innovative programmes.
A spokesman said: "ITV Play aspires to be the gold standard in participation TV and ensure fair play at all levels and we take this error very seriously."
They added procedures had been tightened since the incident.
Along with Rawl plugs and a balaclava, other answers included rubber band, directions, false teeth, contact lenses, driving licence and plane tickets.
The sample answer - revealed before viewers were invited to take part in the quiz - was mobile phone.
Ofcom confirmed this was the first formal breach of the code recorded against ITV Play but warned the channel there should be no further incidents.