No matter how well you think you've vacuumed/swept the floor, the second you sit down, you see a lone Cheerio somewhere.
When you are in the grocery store and a child is misbehaving, you think to yourself, just let me have them for a couple of weeks.
Or at the weekend, when a child cries outside or on the TV, you jump up to swoop it out of it's crib before it wakes the others, and it's only when you are half way up the stairs, you remember you are home alone.