The first that pops to my mind is not powering down and dumping all of my steem when prices were above 5 dollars! Silly me, I actually thought it could go much much higher!
I did sell some near the top, but nothing special, I could have made some serious $$ despite having a relatively small bag of about 10k coins.
Oh well, three years later I (maybe!) got a bit wiser and I think I have selected my main bags much wiser.
What about you ?