Selling my BTC on MtGox for $5 a piece because I mined a block in less than a day on my computer and it seemed like a killer return.
Then probably investing in various ICOs and not dumping the alts when they pumped to get more BTC back.

Oh and spenting my BTC trying to buy miners from Butterflylabs instead of buying the the first Avalonminer. They got to market WAY sooner and by the time I got my piece of crap Butterflylabs miners the difficulty was sky high and I never made back what I bought them for.

So many stupid mistakes lol