What's your actual browser? The most common cause I can think of would be if we think you have a newer browser than you do and serve you too-new code as a result. If that's the case, using an older version in the string might help.

Is there a reason you guys are doing UA sniffing instead of a Modernizr-esque strategy? Would it have too much of a runtime cost? Or is it just about syntax and serving a transpiled blob would make it bigger? Genuinely curious

We do this both for polyfills and for different compiled code in some cases. Since we need this on the first request (and care about perf on that first request), it's less practical to use JS to decide. User-Agent works well in practice.

Chrome Version 60.0.3112.113 (Official Build) (64-bit)

Windows 10