429 Too Many Requests
Slow down: the rate limit burst, and Retry-After says when to come back.
RFC 6585, in 2012, standardized what every API already needed to say: you asked too much, wait. The response may carry Retry-After with the length of the wait and a body explaining which limit burst.
It is probably the most returned code in this collection: every public API rate limit ends in it. Polite scripts read the Retry-After and wait; the others insist — and keep collecting 429s.
HTTP/1.1 429 Too Many Requests
Retry-After: 60
Content-Type: application/json
{"error": "rate limit exceeded"}