4×× CLIENT ERROR
422 UNPROCESSABLE ENTITY
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.
422 UNPROCESSABLE ENTITY Source: RFC4918 Section 11.2
422 CODE REFERENCES
Rails HTTP Status Symbol
Symfony HTTP Status Constant