What website changes or failures are hardest to catch automatically?

[ssshadow-nevermore]

I’m curious how people here monitor client websites beyond basic uptime.

A site can still be technically online while something important has changed or stopped working — a form, booking flow, price block, external widget, analytics script, or other critical element.

If you maintain several websites:

• What kinds of changes or failures do you actually want to detect automatically?
• Which checks have been the most useful in practice?
• What still requires manual verification?
• Do you monitor all sites the same way, or does each project need its own set of checks?
• Have you ever had a client notice an important issue before your monitoring did?

I’m especially interested in the practical side of monitoring multiple client sites and identifying which one needs attention first.