HTTP 400 (bad request) for logical error, not malformed request syntax

Question

The HTTP/1.1 specification (RFC 2616) has the following to say on the meaning of status code 400, Bad Request (§10.4.1):

The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.

There seems to be a general practice among a few HTTP-based APIs these days to use 400 to mean a logical rather than a syntax error with a request. My guess is that APIs are doing this to distinguish between 400 (client-induced) and 500 (server-induced). Is it acceptable or incorrect to use 400 to indicate non-syntactic errors? If it is acceptable, is there an annotated reference on RFC 2616 that provides more insight into the intended use of 400?

Examples:

• Google Data Protocol, Protocol Reference, HTTP Status Codes

Answer 1

Status 422 (RFC 4918, Section 11.2) comes to mind:

The 422 (Unprocessable Entity) status code means the server understands the content type of the request entity (hence a 415(Unsupported Media Type) status code is inappropriate), and the syntax of the request entity is correct (thus a 400 (Bad Request) status code is inappropriate) but was unable to process the contained instructions. For example, this error condition may occur if an XML request body contains well-formed (i.e., syntactically correct), but semantically erroneous, XML instructions.