Working with @nathancolgate has taught me a lot about doing tech support. Thanks to him, I now have two basic principles that I can apply to troubleshoot just about everything:
1. If something is different, something is changing. No matter how similar I think things are, different output = different input. Stochasticity is a cop-out and won’t solve your problem. Find what’s changing.
2. Find what’s changing (you don’t need to know why, just what), and write it down. Be as plain as possible. Google what you just wrote.
80% of the time it works EVERY time.