My uptime checks are all green but users report errors — what am I missing?

My uptime checks are all green but users report errors — what am I missing?

Q: Synthetic monitoring says 100% uptime, yet support tickets say checkout is broken. How can both be true?

A: This is the classic synthetic-vs-real gap, and one e-commerce reader has the numbers. Their synthetic check hit the homepage and a health endpoint every minute: 99.99% green for months.

Meanwhile real-user monitoring showed a 6% JavaScript error rate on the checkout step, climbing to 22% for Safari users after a third-party payment script update. The synthetic check never loaded that script, so it never saw the break. Estimated lost orders before they noticed: roughly 1,400 over nine days.

They added a synthetic flow that actually completes a test checkout with a sandbox card, running every 5 minutes. The next breakage was caught in under 5 minutes instead of nine days.

The follow-up: keep the cheap health check too — it catches total outages faster than a heavy flow. Run both.

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