When you're troubleshooting, or you've been presented with an obvious problem and it's up to you to work out what's going on, it's important not to jump to conclusions. It can be extremely easy to see a problem, such as a website being down, and immediately think, Ah, I've seen this before; it was that time the database had too many rows to count and the website slowed to a crawl—I'll look there.
Lightning does strike twice in computing, especially when root causes haven't been addressed, but it's an anti-troubleshooting technique to immediately assume you know the cause of a problem, based on tentative similarities with past issues.
