Misdiagnosing a public API's concurrency slot limit as a server outage

Symptom

503s. Looks like the server is down.

How the misdiagnosis went (worth recording)

"curl gets 200 but Python urllib gets 503" → concluded it's a client difference → curl immediately 503 too. In reality, the two clients were fighting over 2 concurrent slots per IP.

★ A conclusion from a single observation.

Iron rules

• Never run two collection scripts at the same time
• Check slots via the status endpoint before requesting
• 503 / 504 are noise during normal operation. Absorb with retry + cooldown.
• Mirrors are generally less stable. Fallback only.

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