The most useful mistake I made as a manager was trusting a good team.
I was leading the team at the time. There was a block of work I handed over to people who were experienced, who had never given me a reason to look twice, and who I liked working with. So I stopped doing the verification step. Not as a decision — I never sat down and concluded it was unnecessary. It just quietly fell off my routine, the way things do when nothing has gone wrong for a while.
The work came back wrong. Not catastrophically, but wrong enough that I had to go and explain it, and the honest explanation was not that someone had made a mistake. It was that I had stopped checking.
Delegation is two decisions, not one
The first is who does the work. The second is how you will know it was done right. Most people, myself included at that point, treat these as a single decision — you either trust someone or you do not, and checking their work is a sign that you do not.
That is the error. A good team does not remove the second decision. It just makes it very tempting to skip.
What checking is not
It is not standing behind someone. Verification you agree on in advance is not distrust; it is part of the brief, and experienced people generally prefer it, because it tells them exactly where the line of their responsibility falls. Distrust is verification you do secretly, or that appears out of nowhere the first time you get nervous.
What I do now
When I hand something over, I say the checkpoint out loud. I will look at this on Thursday before it goes out. Send it to me when the draft is done. Or: nothing here needs my eyes, ship it.
That last one is a legitimate answer, and I use it often. The mistake was never having the conversation at all — leaving the checkpoint undefined and then discovering, after the fact, that everyone had assumed someone else was holding it.