Most of people are regretting about transaction that they made on the past , I personally don't regrest any transaction I made , the only mistake I did and I regret is that I wasen't an early adopter , I could've be a millionaire by now most likely if I did 
I would say the worst mistake I made was not believing in it when I first heard of it a few years ago. I would have been rich by now. 
My feelings exactly - I had heard of Bitcoin relatively early on, and even went on to mention it to a couple of friends but, unfortunately, between the little I knew about it at the time, and being too lazy to learn more (and having failed to convince any of my friends to look into it), I ended up forgetting almost all about it until late 2013, when the price took off.
But, even if things had gone differently, I'm not convinced I would be a millionaire by now; between the temptation to sell along the way, or perhaps through investing in some "too good to be true" business/scam, I would probably have lost most coins anyway.
