One particularly amusing bug I was involved with was with an early version of the content recommendation engine at the company I worked at (this is used by websites to recommend related content on the website, such as related videos, articles, etc.). One of the customers for the recommendation engine was a music video service, and we/they noticed that One Direction’s song called Infinity was showing up at the top of our recommendations a little too often. (I think this was triggered by the release of another One Direction song bringing the Infinity song into circulation, but I don’t remember what that other song was).
It turned out this was due to a bug where we were taking a dot product of feature values with feature weights, where the feature value was being cast from a string to a numeric, with a fallback to zero if it was non-numeric, and then multiplied by the feature weight. For the “song title” feature, the feature weight was zero, and the feature value was anyway non-numeric, but even if it were numeric, it shouldn’t matter, because anything times zero is … zero, right? But the programming language in question treated “Infinity” as a numeric value, and it defined Infinity * 0 to be NaN (not a number) [ETA: A colleague who was part of the discovery process highlights that this behavior is in fact part of the IEEE 754 standard, so it would hold even for other programming languages that were compliant with the standard]. And NaN + anything would still be NaN, so the dot product would be NaN. And the way the sorting worked, NaN would always rank on top, so whenever the song got considered for recommendation it would rank on top.
One particularly amusing bug I was involved with was with an early version of the content recommendation engine at the company I worked at (this is used by websites to recommend related content on the website, such as related videos, articles, etc.). One of the customers for the recommendation engine was a music video service, and we/they noticed that One Direction’s song called Infinity was showing up at the top of our recommendations a little too often. (I think this was triggered by the release of another One Direction song bringing the Infinity song into circulation, but I don’t remember what that other song was).
It turned out this was due to a bug where we were taking a dot product of feature values with feature weights, where the feature value was being cast from a string to a numeric, with a fallback to zero if it was non-numeric, and then multiplied by the feature weight. For the “song title” feature, the feature weight was zero, and the feature value was anyway non-numeric, but even if it were numeric, it shouldn’t matter, because anything times zero is … zero, right? But the programming language in question treated “Infinity” as a numeric value, and it defined Infinity * 0 to be NaN (not a number) [ETA: A colleague who was part of the discovery process highlights that this behavior is in fact part of the IEEE 754 standard, so it would hold even for other programming languages that were compliant with the standard]. And NaN + anything would still be NaN, so the dot product would be NaN. And the way the sorting worked, NaN would always rank on top, so whenever the song got considered for recommendation it would rank on top.