What’s the strangest technical SEO issue you all have debugged that had nothing to do with SEO?

Sometimes what looks like an SEO problem turns out to be something completely different CDN settings, server configuration, JavaScript bugs, CMS issues, DNS, caching, etc.

What's the most unexpected issue you've encountered that ended up affecting crawling, indexing, or rankings?

I'd love to hear the story and how you eventually tracked it down.