Oh my goodness... there is one thing that I beat myself up about from my past - leaving my bookbag hanging on the back of my chair during lunch one day while I was traveling in Spain, which ultimately got it stolen. I think the total loss on this mistake was about $2000.00 (and a ton of trouble), and it still can get me heated to this day.
But THIS is something I could never forgive myself for! A contestant mispronounced a word on Wheel of Fortune and it cost him more than a MILLION dollars... what a doof. If he would have just taken a breath and spoke clearly, he could have been a millionaire!