Error code for an invalid POST parameter

Question

What's the proper error code to return when a POST request has an invalid parameter? Say: a form takes data for an event, but the date provided is in the past; or a form takes data for a user registration, but the name provided is a number or any invalid person name.

Answer 1

11.2. 422 Unprocessable Entity

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.

(From RFC 4918)

Answer 2

i would personally opt for: 400 Bad Request

the error should be in the 4xx range which represents client errors, because it is the responsibility of the client to transmit valid data.

anyway, you can have a look at a list of standard HTTP response codes and chose one that seems to fit your need.