Trading bitcoins for a shitcoin during its pump phase. I was left holding the coin for weeks until the price went back to where it was before the pump.
Damn that sucks dude. It really sucks that cryptos had to go through a pump n dump era that would completely ruin the reputation that altcoins had, but some people are more interested in making more money than actually contributing to something.
My worst mistake with Bitcoin was putting some money into a cloud mining site which seemed to just wipe my account at some point.
Not the worst mistake anyone can make, but it's my worst mistake.