Several years ago we moved from a small apartment to a small house (<1800 sq ft). For us, it was neglecting to fully consider all the small to medium costs that came up in the first year. For us, it was garden tools, lawn mower, trimmer, snow shovels, blinds, curtains, paint, paints, etc. Seemed like every weekend we were at Home Depot buying something. These small sub-$100 purchases add up fast.
We also didn't account well for water bills, higher electricity, heating oil, increased insurance.
Edit: or are you talking about the buying process mistakes? If so, don't buy more house than you need. We had a guest room that only saw a guest for 2 or 3 weekends a year! We had a partially finished basement that sat empty for years.