What WordPress maintenance problems are hardest to catch before the client notices?

I’m curious how people managing multiple WordPress sites deal with problems that don’t necessarily trigger obvious monitoring alerts.

Uptime checks are straightforward when a site is completely down, but some of the more frustrating issues I’ve run into or heard about are things like:

• forms silently stopping delivery
• checkout issues affecting only certain browsers
• caching problems
• unexpected noindex or canonical changes
• plugin updates changing something without taking the site offline
• WordPress error states that still return an HTTP 200

For those of you managing client sites:

What are the failures that are hardest for you to catch before the client reports them?

I’m especially interested in real examples rather than tool recommendations alone:

• What happened?
• How did you eventually discover it?
• Did any monitoring tool catch it?
• Is there anything you still check manually because you don’t trust automation to catch it?

Also curious whether this changes once you’re managing 20, 50, or 100+ sites.

No particular product recommendation needed — I’m mostly interested in how people handle these edge cases in real-world maintenance workflows.