A minister inadvertently found himself in the wrong lobby in a Commons vote after a mix-up with a glass of water.
Lib Dem MP Andrew Stunell was filling up a glass from a washroom inside one of the Commons lobbies where MPs cast their votes, when the doors closed.
A spokesman for the MP acknowledged he was "temporarily on the wrong side".
Labour said the government had "plumbed new depths of incompetence" but ministers insisted Mr Stunell had voted with the coalition.
In the House of Commons, MPs vote by walking through one of two lobbies - one if they are voting yes, and the other if they are voting no. Those wishing to abstain on a vote can do so by going through both lobbies.
Lobby doors are closed eight minutes after a vote is called and the only way for an MP to get out is to walk past the tellers - MPs responsible for counting the votes on each side - thereby registering a vote.
Shadow Commons leader Angela Eagle suggested Mr Stunell had ended up in the lobby with opposition MPs on a vote on a Labour amendment to the local government finance bill - which he is helping to pilot through the Commons.
Raising the matter during Business Questions, she asked her Conservative counterpart, Sir George Young, whether Mr Stunell's predicament showed "government incompetence had plumbed new depths".
"Could you confirm that when he was locked in the wrong lobby, the minister bravely took refuge in the toilet - as the Conservative minister barked orders at him through the doorway?
"It says something for the incompetence of this government that it took the Serjeant at Arms to coax the minister out.
"I do think the local government minister has inadvertently revealed the Liberal Democrats' new political strategy. If in trouble, run for the toilet."
Sir George insisted that he had checked the parliamentary record, Hansard, and Mr Stunell had voted "the right way".
He told MPs: "I understand that after so doing, as an act of generosity he went to refill one of the (water jugs) so his fellow ministers on the bill would be refreshed.
"He was in fact in the opposition lobby. I understand there were fraternal greetings. I'm grateful he managed to emerge from that lobby unharmed."
A spokesman for Mr Stunell, MP for Hazel Grove, said he had voted against the Labour amendment "as he was supposed to" and had gone to get a drink of water for a fellow minister when he found that lobby doors had been "locked".
"He was temporarily on the wrong side but sanity prevailed and they let him out," he said.
Such mix-ups happened "more often than you think", the spokesman added, accusing Labour of putting a "bizarre spin" on the episode.