Not precommitting to be on my own before making a major life decision.

I once bought something in an New York shop through high-pressure sales. I looked at it and said something about how I would like to have it but I couldn't nearly afford it, and he asked me how much I would pay for it. Foolishly, I named a price; he looked insulted and said that it was far too low. I tried to explain that that was what I meant, that I couldn't afford it at any reasonable price, but he skilfully turned it into haggling, and I walked out with the thing and considerably poorer. I then resolved never to buy anything expensive without leaving the shop first, so I could just walk off if I changed my mind.

Many years later, I met up with my girlfriend's girlfriend for dinner and drinks so we could discuss whether it would work for her to move in with us. There were a lot of warning signs that it wouldn't, to say the least. I pressed her on things that were worrying me, and got wholly unsatisfactory answers. But we very often had good and enjoyable conversations, and this was one of those times. So at the end she sort of said "OK, that's all great, shall we announce online that I'm moving in?" and it wasn't easy to say no. The result was very costly for all of us; it was definitely the biggest and most predictable mistake of my last decade.

Going into such a conversation another time, I'd have said well in advance that I wouldn't be making any decisions until the next day, when I was on my own. I think there's every chance that that simple precaution would have saved untold suffering and money for all concerned.