428Precondition Required
The server insists the request carry a condition, so that two people cannot silently overwrite each other.
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An API protecting itself from lost updates. It refuses an unconditional write and requires an If-Match header naming the version you believe you are editing. If that version is stale you get a 412 instead, and nothing is overwritten.
The fix from a client is to fetch the resource first, keep the ETag it came with, and send that back on the write. It costs an extra request and removes an entire class of silent data loss.
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1xx Informational (4)
2xx Success (10)
3xx Redirection (8)
4xx Client errors (31)
5xx Server errors (16)