Planned risk assumes a functioning market. Worst-case loss assumes it is not. For a stock position the realistic worst case is the position value in a fraud or a halt-then-reopen; for a short it is theoretically unbounded; for a naked option seller it can exceed the account.
Use a stress multiple instead of imagination. A common desk rule is to price a 3x-stop-distance adverse gap on single names and check the result against the account: if 1% planned risk becomes a 4% actual loss, can you take three of those in a week? That is a stress-testing question with an arithmetic answer.
The point of the exercise is sizing, not fear. Positions whose worst case is survivable can be held through anything; positions whose worst case is not survivable must be smaller, defined-risk, or skipped entirely - no matter how good the setup looks.
Related: gap-risk, largest-loss, stress-testing, catastrophic-stop