What is the difference between a 500 error and a 400 Bad Request error, and when would each occur in a C# web application?

A 400 Bad Request error indicates that the server couldn't understand the request due to invalid syntax or missing parameters – it's a client-side error. A 500 Internal Server Error indicates a server-side problem that prevented the server from fulfilling a valid request. A 400 error occurs when the client sends incorrect data, while a 500 error occurs when the server encounters an unexpected error while processing a valid request.