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Migration of maps to upload caches resulted in query parameters being stripped, breaking some subservices (task T166735).
This is a step by step outline of what happened to cause the incident and how it was remedied.
- There is never enough monitoring.
- An important change was considered ops-only, was never communicated to its product team, neither beforehand for coordination nor afterwards for a quick check that everything still works.
Explicit next steps to prevent this from happening again as much as possible, with Phabricator tasks linked for every step.