What's one thing you wish you'd known before your first live trade?

One lesson per reply, please. Short answers are fine: the best threads like this read like a list of hard-earned warnings.

A few that new traders run into again and again, to get it started:

• A stop-loss can fill at a worse price than you set when the market gaps or moves fast.
• Swap is charged for every night you hold a position, and a triple swap is usually charged on one weekday to cover the weekend.
• Spreads on many pairs widen around the daily rollover, shortly after 5 p.m. New York time.

What would you add?