Rule one - treat all microphones and cameras as live.
If there is a single political campaigner who has never walked away from a voter thinking or muttering unbroadcastable thoughts, then I'd like to meet them. A day away from the final, stage-setting debate and a week away from polling, the stress he is under is massive and he will be close to exhaustion, with his premiership at risk. He was being followed by Sky and was understandably concerned about how they would cover the encounter - ironically, I don't think that it would have played badly before his comments were broadcast.
I've read differing stories as to what the lady involved actually said, but the important thing is how he's reacted since - he's apologised publicly and in person to the lady concerned.
The snap polls seem to suggest that his apology will suffice - 58% think so and another snap poll says that 74% wouldn't change their vote as a result of the kerfuffle. He's human and he's made a mistake for which he has now apologised. Interestingly, most other politicians are holding back on comments on the issue - they understand.
Clearly, this will provide food for the next 24 hours of the news cycle and fuel the pro-Tory media's attacks on Labour, but then we have the final debate, focussing on the biggest issue for us all - the economy and that will set the headlines for the weekend and he run in to the finishing post.