417Expectation Failed
The server cannot meet the expectation set in the request Expect header.
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Effectively only when a client sends Expect: 100-continue and something in the chain will not honour it. Rare, and normally resolved by not sending the Expect header at all.
Older proxies are the usual culprit, since the expectation has to be understood by every hop between client and server. If a request works directly but fails through a proxy, the Expect header is worth removing first.
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1xx Informational (4)
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3xx Redirection (8)
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