I would say that my biggest mistake was to sell bitcoins that i have been stashing for a long time @~200 $, when the price later on jumped to
~250 and later on to ~1200. I couldn't believe my eyes, but the only good trade decision i did was not to rebuy, that would end badly.
I believe most of people made the same mistake here.
Ahh. You made a mistake selling Bitcoin early. I hope that you will hold onto your Bitcoin today and wait for the price to increase. No worries, if I were you I will also sell my Bitcoin when the price is going up everyday - Bitcoin 2013 bubble eh.
The worst mistake I did with Bitcoin is playing gambling and lost almost 6 BTC just in minutes.Gambling is addictive and in the end it let you without money.This is biggest mistake I did since I started using Bitcoin.
Six Bitcoin is a large amount. I wonder what was the value of one Bitcoin the day you lost your six Bitcoin.
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.

Well. Many people said this in this thread and... I have no comment on it anymore. :/.