What is the biggest investing mistake that you've experienced and would never forget? Could it been 100% avoidable or is learning the hard way unavoidable?

Yes, I dabbled in CFDs and made a killing of nearly 6 figures. I then became over-confident/emotional and lost it all including my capital through a series of bad decisions. You can learn from other people's mistakes (like mine) to get a headstart. But just remember that any investors can make mistakes - even Warren Buffett with his Tesco purchase.