What’s the most frustrating thing that’s quietly broken on a client site without you knowing?

Hey everyone,

I’m building a website health monitoring platform for website owners and web agencies and want to make sure I’m covering as many bases as possible.

We monitor loads of health metrics, such as domain and SSL expiration, uptime, basic technical SEO and AI readiness checks, security, pagespeed, server monitoring, cron job monitoring and lots more. But I'm sure there are probably more edge-case scenarios we could extend the platform to help guard against.

What’s the most frustrating "silent" issue you’ve had happen on a live site?

Thanks,
Tom