For my latest hobby, amateure astronomy, I bought a small "starter" telescope. Of course new, because I didn't know how to judge the quality of a second hand. I bought a separate mount, but bought the cheapest one I could afford. Half a year later I already wanted a bigger telescope that shows more deep sky. I put it on the existing mount, but it is way too light for the new tube. I should so much have invested into a heavier mount to start with. And maybe into a bigger telescope to start with. The one that I have now is a nice compromise. I could have bought that from the beginning. It would have been cheaper than buying two.
Our current house was also a financial mistake. This was just before I discovered this website. I wanted to move out of our old house which needed a lot of renovations. We found another house in the same village. The view of the new house is to die for, even better than the view from the old house. But we paid a high price for it and I could have left the money in index funds where they were. The new house is badly designed in matters of energy use, which is dumb when you live in a cold country. It was on my list to buy the perfect house: great view, large rooms, no traffic noise, lots of nature close by, low energy. We fell for the first 4 things. The other negative thing about the house is the terribly steep roads in bad condition that lead to it. For us with a Subaru and a Suzuki it is not a big problem. We can drive up there in winter. But there are many others using the same roads without a 4x4 who get stuck and block the roads. But when selling the house, it will be a big downside. Like it was with our old house, which had a similar road, but with less traffic on it. Only positive thing about the house: our not-so-popular community will be combined with the neighbouring popular community. That will make our house more worth, or at least more attractive for more people. Then hopefully we will not lose too much money on it. But in general, I think we paid such a high price that I am less and less convinced that we will ever get it back at a sale.