Due to the semi-recent changes in how we gather the logs for analytics processing, URLs containing UTF-8 characters were failing to be properly logged. So, for example, if you had a URL containing "?utf8=✓", the request would work just fine, but our background processing to log that in our analytics db was failing. This was due to how nginx escapes and logs special characters.
Surprisingly, this hasn't actually affect much of our traffic. It's only been cropping up in our error logs a few times a day, but it's still definitely something we want to fix.