What rule do you think most traders misunderstand?

For me, it has to be the daily drawdown rule.

A lot of beginners think it’s just a flat percentage of their starting balance, but they get caught completely off guard by how simulated equity trailing works or when the daily reset actually happens (especially with time zone differences). It's incredibly frustrating to see people lose accounts they were actually in profit on because they didn't realize their open equity trailed.

What rule did you have to learn the hard way, or what do you constantly see other traders tripping over? I appreciate your honest opinions.