How to handle errors? What is the best practice? What sort of message user should see?

The status codes are made accoridng to the following recommendations:
http://racksburg.com/choosing-an-http-status-code/

Examples how user can fix an error 400 user entered email without @ sing: he should enter a valid email 400 user entered literals in a field for numbers: he should enter a valid number 401 user entered wrong password: he can fix it by entering right password 410 application is out of date: user has to update it 422 receipt is not printed: user has to print a receipt first Examples how developer can fix an error 400 user entered email without @ sing: developer forgot to add do an email validation to the form 400 user entered literals in a field for numbers: developer forgot to add do an numberic validation to the form 403 an app lets user to operate over server without permission from the server: developer forgot to add proper validation when signing in 404 user requested data that is missing on the server: an app contains a defect (bug) that lets user to operate with deleted data 405 syntax mistake in communication protocol: a defect (bug) in an application 409 logical mistake in communication protocol: a defect (bug) in an application Why 400 is at both left and right side? Some applications, for instance thin clients (browsers) marked here as “TC”, might prefer to let server to do a validation upon entered data, then process error message and return a valid error message to the user so user fixes it.

Some other applications like rich user interface applications such as GUI, might prefer to process errors on the GUI form and when processing is done, send only valid data. In this case if server returns an error it indicates that developer missed some validation during form development. Which leads to an application error. Why developer can not fix some errors? 503: server is under maintenance or overloaded. 500: a defect (bug) inside the server. How to handle such situations? Server support team monitors server 24/7 and does all the best to avoid any 5XX errors. Feel free to contact support team if you experience 5XX during significant period of time. Where can I find full list of all the errors? Here . Important Scheme above is a rough scheme. Study all the errors carefully and make wise decisions based on your business logic. Sometimes (for instance for background processes) there are no user communication at all. And so on.