The obvious one, not buying bitcoin when it was dirt cheap. Then buy at $100, and offloading them at $450, thinking I had scored a fantastic deal. Hindsight is wonderful, isn't it. Oh! How I wish, it does not work like that. Not even the longest term holders dreamt it will be like this.
That was my biggest regret to, turning down and disregarding bitcoin opportunities in those early times. Even when it was introduced it again to me by my friend I didn't move asap because I was still skeptical about different things about it but as I go deeper the more I realized that I just wasted such golden opportunity.