Speaks to the danger of building your business on an API that can change at someone else's whim. Also, the importance of being very thorough in your testing.
Still, I'm glad you were able to catch it in time and reverse the trend.
It was a mistake on my part, the API itself didn't change in any unreasonable way (I shouldn't assume that array prototypes have no extra members). A lot of revenue was lost, but my feeling about it now is mostly relief, as I wasn't sure if I'd ever find the cause.
I think you're showing your true hacker colours because you also seem excited about understanding the bug for its own sake. :-) I would be too.
BTW: I actually think an array changing its behaviour is an unreasonable change - for languages like C and Java - but a reasonable one for dynamic languages. It reminds me of a comment someone made about the "C# and Java Weekday Languages, Python and Ruby For Weekends?", that C#/Java are less fun to code in, but more fun to maintain, and the same features are responsible for both effects.
I was about to chime in "I feel for you, something really similar happened to me" and then realized you had already cited that exact bug in your blog post. I don't know whether this makes me happy or sad.
The great thing about losing potential revenue is that there is an infinite amount of it out there. I know that is cold comfort but it helps me sleep soundly at night, in the face of some bugs which have been fairly costly.
I caught a subtle one last night which would have been a real, real doozy. I was busy writing additional documentation for my A/B testing framework and then wondered, hey, why does this thing I'm recommending sound very different than what I remember actually implementing?
Then I thought "No, wait, that's preposterous. If I had actually implemented that, I would have gotten subtly incorrect garbage data which would have... passed... all my... tests... until deployed live... oh dear."
On the plus side, this time it only cost me a weekend full of analytics data rather than $10,000, but if I had started "improving" my site based on the borked data shudder.
Still, I'm glad you were able to catch it in time and reverse the trend.