I usually extend my hour in case that happens. It means noting the time when the maximum of each species occurs as well as the number. This morning I watched from 11 till 12; the maximum robins and sparrows were at 11:15 and the maximum blue tits at 11:45. Others were coming and going all the time. So I carried on till 12:15, and if anything exotic had turned up, I could have said my hour was 11:15 to 12:15. But of course it didn't!
I only started doing the BGBW last year and was disappointed to count one blackbird (m) and one blue tit, who snuck in on the 59th minute. I'm sure that they were waiting for me to stop watching! Usually, I would see loads of different birds in just half an hour. Thought it might be the time of day I chose - 10am-11am, so decided to do it differently this year. It was a bit better with 6 different species, but only 7 birds (2 x blackbird). However, normally, I'll be watching 3-4 wood pigeons at the same time as up to 6 collared doves, with up to 4 blue tits flitting in and out of the bushes and at least one robin and the resident pair of dunnocks and blackbirds. I'm utterly convinced they know that they are being counted and decide to evoke the "I'm not a number" attitude. :-)