Experience is a good teacher, but she sends in terrific bills– Minna Antrim  There sure are a lot of bills:  not selling and taking a profit even if small, not buying from fear or lack of understanding, buying or selling based on following your guru (forgetting they may just be you with a big pay check), understanding the Market or particular stock from a cm away and 100s of miles away.

Mostly, not believing what you see and feel but believing talking heads and other pundits.