What do you check before you place a trade, and what makes you skip one?

Before I get into any setup, I find it useful to run through a short checklist: what is the reason for the trade, where would I be wrong, how much would I lose if that happened, and does the position size fit my plan. Writing those four answers down often shows whether an idea is a considered one or just a feeling about a chart. Here is a made-up example with round numbers. Suppose an account of 10,000 and a rule that no single trade risks more than 1 percent, which is 100. If the distance between the entry and the point where the idea is proven wrong is 2 units of price, the position can be sized so that hitting that point costs about 100. If the distance is 10 units, the same rule means a much smaller position. The idea itself did not change, only the size, and that is the part many people skip. I am also curious how often a checklist leads people to skip a trade they would otherwise have taken, and whether those skipped trades later turned out to matter. What is one check in your own pre-trade routine that you would not want to drop, and why?