The Prime Minister was holding her own in a head-to-head Q&A against Labour leader Jeremy Corbyn in the Battle for Number 10.
But during the public grilling, one audience member was less than impressed with her response to questions on the NHS.
The audience member could be seen mouthing "that's b*******s” twice during the first joint appearance between Mrs May and Mr Corbyn.
His inaudible comments are believed to have been in response to Theresa May’s claim she was putting “more money into the NHS”.
Twitter users followed suit as they mocked the Prime Minister
Twitter users followed suit as they mocked the Prime Minister – and called the man a "legend" and a "hero".
One Twitter user wrote: "Theresa May being upstaged to man mouthing ‘b*******s’ on #BattleForNumber10".
Another added: "Best bit of the Corbyn & May show so far: man in the audience mouthing "that's b**s" to Theresa May's answer regarding the NHS."
The PM also faced questioning on the Tories' proposed social care reforms during her first TV debate of the election campaign.
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Jeremy Corbyn during a visit to Hackney Marshes Football Pitches, to highlight Labour's manifesto commitment to ensure 5% of the Premier League's television rights income is diverted to the grassroots game, during a General Election campaign
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The PM also faced questioning on the Tories' proposed social care reforms
The Prime Minister was challenged over her plan to protect the value of inherited estates by imposing a cap on social care costs.
She said: "We are forward a proposal which means that people don't have to sell their house in their lifetime to pay for care.
"It means they can pass on savings to their children. And it means there will be that cap. But it's fair across the generations."
The Prime Minister was challenged over her plan to protect the value of inherited estates
Mrs May also said previous social care plans protected wealthier pensioners but did nothing for those on modest incomes.
She told a studio audience: "The proposals I have put forward actually do that.
"They protect pensioners on modest incomes and crucially they ensure we have a long-term sustainable solution for social care."