Sometimes, but not always... For example, the Edge bug was at [1]. It didn't reproduce when the devtools were open, it only happened if the optimizer tried to

Sometimes, but not always... For example, the Edge bug was at [1]. It didn't reproduce when the devtools were open, it only happened if the optimizer tried to optimize it which was non-deterministic, and it would only trigger in some cases in pretty weird spots with a lot of supporting code. Feature detecting that would be a nightmare, but UA detecting it (since I now know the exact versions impacted) is super easy and simple and the fix is extremely small. I don't have that in our codebase any more, but it was a perfect example of a bug which can't easily be feature-detected. [1] https://github.com/microsoft/ChakraCore/issues/1415 https://github.com/microsoft/ChakraCore/issues/1415