Well worst mistake would be to invest too much time and resource to gambling. Let's face it gambling is a game of chance, there's no guarantee in gambling so making a bet that would either make or break you financially and emotionally would be a very big mistake
Something we all regret. I also never gave any importance to my studies as a student due to gambling ( non btc ) and now regret why i did so. Anyways I have a settled life so yeah when I look back, I feel that was my biggest mistake.
Non technical gambling mistakes we can say such as what you stated.
For a worker it is interfere your work , for a student it is make you looks not smart because gambling is wasting time.
Regret why you did not take study seriously.