A fact-checker at The New Yorker wanted to know how journalist John McPhee was so certain that a river he’d been floating down was the exact temperature he reported.

Why, he’d simply hung a thermometer off the bow of his canoe and looked at the results.

I used to tell that story to my journalism students as an example of fact-checking at its finest. I also used to spout legendary crime reporter Edna Buchanan’s three rules for reporting:

Never trust an editor.

Never trust an editor.

Never trust an editor.

In other words, don’t count on someone else to catch your mistakes.

I’ve changed my tune on that one lately, having just survived a rigorous, occasionally sweat-inducing fact-checking experience. For the March issue of O, The Oprah Magazine, I wrote an essay about a crusty newspaper copy editor I used to work with named Lynn Forbish. As her mind slipped into dementia in 2006, she asked me to tell her story — before she forgot it.

A stickler for details, Lynn was the kind of copy editor who had no problem getting a reporter out of bed at midnight to check a fact. So it was fitting when an O fact-checker tracked me down on vacation recently to double- and triple-check several details in the story, which had already been through four editors at least.

Lynn’s relatives recalled some details differently than I had, including when exactly Lynn’s mind began to falter. Before her 61^st birthday? After her 62^nd? (It was sometime between the two.)

Which paper was it that she’d worked for in St. Pete – the Times or the Evening Independent? (Both.)

Another editor wanted to me to verify the anecdote about Lynn fighting a bull in Spain. Can a tourist really do that? Step into a ring with an angry bull?

I had relied on a family member’s recollection for that detail. But it was one of those stories that had been told and retold so many times that it turned out no one really knew if or how Lynn fought the bull.

She had, albeit a small one — only it wasn’t in Spain, it was Mexico, according to the newspaper article we finally tracked down, written by Lynn herself. As a wonderful byproduct, that fact-finding mission produced a keeper of a photo of Lynn brandishing a red cape — resolute, brave and absolutely beaming. (I considered it bonus proof that the old gal really had faced down a bull.)

A reporter now for 25 years, I’m sorry to say that most newspapers simply don’t have time to fact-check with the rigor of a monthly magazine — although a certain British copy editor named Suzanne likes very much to keep me on my toes.

I like to think the magazine pieces I’ve written in recent years have had a positive spillover into my daily work. Surviving the fact-checkers has made me try harder to prevent those humbling, late-night “How do we know this for a fact?” calls from Suzanne.

After my November nail-biter of a trip to Haiti, I had to describe what it was like to navigate through angry protests and roadblocks as the medical team I covered fled cholera-decimated Limbe. But were there six roadblocks or seven? And at what point did we arrive at the scariest stop, the one with the blocked-off bridge and the man who clutched both a machete and a stick as he ran straight for us?

In the front seat of our truck, I was so scared I only took notes between roadblocks, after the danger was passed. And you try making sense of notes written while riding on dirt roads with mattress-sized potholes at the same time you’re thinking, man, I really wish they had guns instead of machetes.

At least with guns, death might be mercifully quick.

It took several follow-up interviews with team members before I felt comfortable describing the escape in detail. The bridge roadblock was the fourth, we all agreed. There were six roadblocks in all.

Part of the medical team, in all its post-rescue glory.

The whole ordeal lasted how long? Not one of us had a clue, a detail that spoke volumes about how the brain processes trauma. Could have been 20 minutes, could have been 90.

When is a fact really a certifiable fact? What do you do when people remember things differently, as they ultimately will?

When you’ve been reporting for as long as I have and in basically the same community, it can be tempting to not make that extra phone call, not challenge that source you’ve been calling on for years, not give that piece you could’ve just written in your sleep a 16^th or 17^th going-over before you turn it in.

For my recent series on the politically contentious issue of Lyme disease, I knew I couldn’t rely on old habits when I interviewed a doctor who greeted me with, “I’ve been looking forward to this as much as a root canal,” then proceeded to pick apart my every question. (When I casually asked what tick-precautions I should take when I hike up Mill Mountain, he shot back: “Well, are you in shape?”)

Which is why I decided to record every interview for the series — something I rarely do — even though the transcription time doubled my work.

Should anyone question the validity of my quotes, I wanted my own thermometer in the water, my McPhee-level proof. (See a fabulous interview with the literary journalism master in this Paris Review.)

McPhee has written that the worst checking error is calling people dead who are not dead. I’m happy to say I’ve never committed that sin, though I once used the wrong pronoun on a second-reference to a person, giving him — or her; I forget which — a print sex change. Once when was I was very young and rushed I made reference in a profile — of an English professor, no less! — to the writer William Thoreau. Ugh.

It’s the things you think you know that get you into trouble every time.

Long live the fact-checkers.