Ideally, we would have installed wood floors, but they were a bad choice at the time for several reasons. Before we chose the color, we talked about the possiblity of stains, but decided it would be okay until we moved (since we don't plan on living here indefinitely). Alas, it looks terrible, although we don't wear shoes in our home, vacuum often, and have had it professionally cleaned several times. It's the one major design choice I regret in our home, and I regret it weekly. Carpets aside, the rooms are lovely, but the stains drive me bonkers.
So I'm curious — What's your biggest design regret? A paint color you decided to live with and now loathe? A dresser you didn't buy at an antique fair and wished you had? Your regret can be big or small, I'm just in the mood for a little commiseration.