What is optimal HTTP response Code when not reporting 200 (everything OK) but error in input?
Like, you submit some data to server, and it will response that your data is wrong
using 500 looks more like Server Issue
using 200 with warning/error response text is bad (allowing caching and everything is not OK)
using 204 and returning nothing, is maybe good (but well supported?)
using 404 is wrong if requested path (script) is available and in proper place