Going to the wrong dentist for some complex implant work. Researched him carefully - he checked out. But his treatment plan didn't work: created a problem I can never fix that will likely result in dentures eventually. Saw other specialists who confirmed it.
Broke my heart as I am diligent about hygiene and spent a lot of money on my teeth over the years.
Laws in my state make it virtually impossible to sue dentists and win - there are many sad stories far worse than mine. Attorneys are rarely interested in taking a case: usually harder to win than med malpractice and no big settlements.
I've made money mistakes, but I recovered from those. The dental thing is #1 regret. But as someone said earlier, I made the best decision at the time, given the info I had.
And reading about the earlier poster in that horrible auto accident certainly brings important perspective - so sorry for that nightmare you experienced. It is so scary how a few seconds of bad timing on an otherwise normal day can change or end a life.