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500 Internal Server Error
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501 Not Implemented
This error should be very rare in any Web browser. It is more likely if the client is not a Web browser - particularly if the Web server is old. In either case if the client has specified a valid request type, then the Web server is either responding incorrectly or simply needs to be upgraded. -
502 Bad Gateway
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503 Service Unavailable
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504 Gateway Timeout
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505 HTTP Version Not Supported
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506 Variant Also Negotiates (RFC 2295)
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507 Insufficient Storage (WebDAV) (RFC 4918)
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509 Bandwidth Limit Exceeded (apache bw/limited extension)
This status code, while used by many servers, is not specified in any RFCs. -
510 Not Extended (RFC 2774)