One of my son-in-law's favourite expressions is: "we could all be chairman of the hindsight committee". How true. Hindsight is a terrible, often painful, thing. With hindsight, I would never have:
Failed to make sure that the bannister in my son's house reached the top of the stairs when I was going down them in the dark (result: fractured spine, and several weeks in hospital).
Bought that appallingly expensive blue top, with sparkly bits, which I've only worn once (and then only because I felt I had to, having spent so much on it).
Bought a terrifying thoroughbred mare called Patsy, who threw me around the Wiltshire hills for about four weeks before I admitted defeat ("you should have bought me," said Titch. "You were only a baby then," I told him. "So I couldn't have even if I'd known about you.").
Gone to university, which I loathed (long story).
Allowed Wilbur (our exceptionally stupid ginger cat) to escape when we last moved house. We never saw him again.
I could go on and on, but I won't, for I have a feeling hindsight might kick in again afterwards, muttering about wasted time...
(What single thing would you change, with hindsight?)