"What if this is the one time?"

Every rechecker has a story. The morning, years ago, when they went back and the iron was on, or the door was open, or the email had the wrong attachment. That story is retold every time the doubt arrives. It's the one time, and it proves the habit was worth it.

I want to take the story seriously and then look at what it proves.

It proves that things are occasionally left. Of course they are. People are tired and distracted and the morning is full. The question is which check catches them. Think about the morning of the story. You went back and the iron was on. Why was it on? Because you hadn't turned it off, which means you hadn't done the once, or had done it while thinking about something else. The recheck caught what the first check had missed, because there hadn't really been a first check. That's an argument for doing the once properly. It isn't an argument for rechecking a check that was done.

Now look at your tally from chapter 1. However many marks it has, the number of times something was wrong is close to zero, and if it isn't zero, look at whether the thing that was wrong had been properly checked in the first place. In my observation, the story of the one time almost always turns out to be a story of a missed once, and never a story of a good check that somehow came undone.

There's also the arithmetic of the story. One vivid case, years ago, against thousands of rechecks since that found nothing. Rachman's multipliers explain why the one case weighs so much. It raises the perceived probability of harm, and a raised probability keeps the checking going, and the thousands of clean checks don't lower it again because clean checks aren't memorable. You remember the open door. You don't remember the four thousand locked ones. So the story grows and the evidence against it doesn't, and the habit feels justified every morning by an event from years ago.

I'd put it this way. The one time is real, and it's the reason to check once, well, every time. It's not a reason to check four times, because the fourth check protects you from nothing the first one didn't, and it takes away the clear picture that would have let you leave.