Theresa May was teased by Labour MPs after declaring Brexit would mean leaving the UK instead of the EU - before Jeremy Corbyn focused on buses at Prime Minister's Questions.
Mrs May laughed as she was mockingly cheered by the Opposition for the slip-up, which came as she praised BAE Systems's multibillion-pound Australian warships contract as a sign of closer ties with allies post-Brexit.
But Mr Corbyn was also heckled with shouts of "taxi" from the Tory benches as he used his six opportunities at Prime Minister's Questions to challenge Mrs May over the bus industry, with no mention of Brexit.
He declared a Labour government would "save the bus industry" and offer free fares to younger people as he criticised deregulation.
Before the leaders' exchanges, Conservative MP Rachel Maclean (Redditch) hailed the warships contract as another "great victory" for the UK following England's World Cup victory over Colombia.
The Prime Minister, in her reply, said: "The scale and nature of this contract puts the UK at the forefront of maritime design and engineering, demonstrates what can be achieved by UK industry and Government working hand in hand, and it's the start of a new era in strategic defence industrial collaboration between the UK and Australia, which will be reinforced by the forthcoming defence industrial dialogue.
"As we leave the UK, as we leave the EU..."
Mrs May paused and laughed after realising she had made a mistake, adding: "As we leave the European Union, we have an opportunity as the UK to build on that closer relationship with allies like Australia, and that's exactly what we're doing."
On buses, Mr Corbyn said: "With fares rising above inflation, passenger numbers falling and services being cut, does the Prime Minister accept her failure on yet another public service - the buses?"
Mrs May replied: "I think we should look at the responsibilities that local authorities have up and down this country for the buses."
Mr Corbyn claimed the Government since 2010 has cut 46% from bus budgets and there has been a 10% drop in passenger numbers among elderly and disabled people.
He said: "Her Government belatedly committed to keeping the free bus pass, but a bus pass isn't much use if there isn't a bus."
Mr Corbyn later added there are "26 million fewer journeys" made across the north of England and the Midlands under the Conservatives, adding: "So much for a Northern Powerhouse and a Midlands Engine."
Mrs May said the number of people using buses under Labour London Mayor Sadiq Khan has decreased before saying Tory West Midlands Mayor Andy Street had extended free bus fares to apprentices and students.
Mr Corbyn replied: "It will be a Labour government that saves the bus industry and a Labour government that gives free fares to under-26-year-olds."
He criticised deregulation, saying it had led to fares increasing faster than inflation, ridership falling and the private bus "monopolies" making a profit of £3.3 billion since 2010.
Mr Corbyn went on: "That's what the Tories give you in public transport."
Mrs May said local authorities have some responsibilities and capabilities in subsidising bus routes and fares, adding metro mayors have the powers too.
She went on: "I'll tell him what's happening in the Northern Powerhouse and the Midlands Engine - more investment in our public transport, more investment in our roads, more investment in the infrastructure that brings jobs to people in the north and across the Midlands."
Mr Corbyn finished by saying bus services are in "crisis", saying it is bad for the UK's climate change commitments, for air quality and that Mrs May needed to recognise it was the only mode of transport available to some people.
He added: "End the cuts to bus budgets and give councils the power to ensure everyone gets regulated bus service wherever they live."
Mrs May said: "I will take no lessons from (Mr Corbyn) in devolution to local authorities."