Sold 14ETH for 80ish pounds thinking I was a genius and there was no way it could go higher. 14ETH atm is £6.8k and climbing.
Lesson learned: The crypto sphere has enormous scope for growth; don't underestimate its potential.Panicked when Ark first started dropping and sold it all, then bought it all back shortly after when it started rising again. 2k Ark gone for no reason

As of now, that 2k is worth £6.5k.
Lesson learned: Just because a coin is going down doesn't mean it's bad.Spent the £600 pounds I was going to use to buy Dash on a plane ticket. Wasn't really my fault as this was a family obligation but I could have put my foot down. When I saw it skyrocketting I didn't buy back in because I was salty. As of now, that decision has "cost" me £66,500.
Lesson learned: Don't get salty just because you missed an opportunity to buy in early. This helped me recently. I meant to buy Iota one day but ended up getting delayed, and it went up by 40% overnight. I was feeling bitter, but I still bought in. Since then what I put in has almost tripled.There was also PivX... It sounded promising to me and I was going to buy a few hundred pounds' worth one night, but I'd read an article earlier saying to not buy coins on impulse when you're tried. I thought it made sense and decided to ignore PivX. Shortly after, the anon-coin frenzy kicked in and PivX exploded. It went up by (IIRC) x100. As of now it's gone up by over 200x. I'm not even going to calculate how much that would be.
Lesson Learned: None. Ouch.