A week or two ago, as I was reading the corrections box on page A2 of the Post, I thought to myself that it had been a while since I’d had to run a “cx” on my own work. I credited having a saner workload… and then wondered if I was due for an error anyway.

Turns out I was. I left a simple but stupid mistake in the feature I wrote for Ars Technica about the unlikely success of indie ISP Sonic.net–listing the price of this provider’s 1 Gbps fiber-optic service as $79.99 instead of $69.95. My editor at that site unknowingly put that price in the headline and therefore, as you can see from the link above, memorialized it in the story’s address too.

Sonic’s CEO Dane Jasper spotted the mistake within a few hours of the story’s appearance and notified me in a Twitter message. I e-mailed my editor, who had it fixed minutes later… and then I could get on with my “how could you?!” follow-up. (Figuring out how an obvious error wormed its way into a story is more constructive than walking around and cursing at yourself.)

The Versions feature of Apple’s OS X Lion, as seen at right, revealed that I didn’t add the price of the service to a draft of the story until Feb. 13–weeks after I’d started my reporting. Then I typed in the wrong number and kept using it from then on.

The Evernote file with my notes from interviewing Jasper and some of his customers had never included that price. My e-mail showed that I did mention the right number, rounded up to $70, in my pitches to Discovery News (which should soon post my take on what a connection that fast feels like) and then Ars–but had subsequently written “$79.99” to one source on Jan. 25 and to another on Feb. 11.

It appears that this number lodged itself firmly in my brain and never got out.

I can’t blame any of the usual excuses, like not having time to verify things. I spent weeks on this piece and checked just about everything else–the copy I filed had a link back to the City of Sebastopol’s demographics page confirming its population as 7,397. But for the number that wound up in the headline, the most important one in the piece, I never thought to link back to the relevant press release on Sonic’s site. (Doing so would have also avoided the confusion expressed by one Ars reader: Why isn’t this service listed among Sonic’s services?)

So that’s this week’s lessons re-learned: Put the important numbers in your notes at the start instead of leaving them in your head, and link to your sources, so readers don’t have to take your word for things. Or just don’t be a flake.

(I’ve yet to see any readers call me out on this. But I’m irked anyway, which is why I just devoted almost 500 words to unpacking my mistake.)