One user's page froze, and every dashboard stayed green. It was a long-lived single-page app, the kind of tab someone keeps open for hours. A customer writes in: yesterday the page locked up and they lost work. You open your monitoring to pull that exact session, and it is not there, because your replay tool sampled it away and it threw no error to catch on.
Modern error-monitoring and real user monitoring (RUM) tools are good: real Core Web Vitals in the field, session replay, per-session drill-down, and rage-click and dead-click detection all ship today. Three failures still fall through, though, and this post is how we caught them, with a per-session recorder built on tooling most teams already own.
TL;DR
The session behind a complaint is often missing: replay tools sample sessions for cost, and silent failures throw no error to trigger capture. Keep every session on infrastructure you own, and any complaint stays traceable.
Scroll jank and sustained drag stutter go uncaptured, because these tools detect frustration through clicks only. Each needs a small detector that samples the gesture with timestamps.
