I had an interesting mystery the other day. I work in the UK, but I'm working remotely from the US for a while. First day, I made some changes, ran the tests and they failed. Weird part was the failing test was for a component I hadn't touched. I took a closer look, and realized it was a date off by several hours. The test was checking that a passed in date appears in the output. But it was creating the date by parsing a string. The library I was using defaults to local time, but the component uses UTC. So, I had inadvertently created a unit test that only passes when run from UTC. But I had never noticed before because my work is in that timezone. Yikes!