Form Error message in Application

Hi,

i got curiosity on the Errors we see on a form. One way is to display error messages intentionally if we want to display any specific Error. The other is although a basic Application is used and it has 2 lines of code. There is a c BSFN used and because of the Logic the BSFN will go in error. Although we do not have a return paramater which says that it went into error still the Application displays error message.

What is happening here behind the scene? Any inputs will be helpful to understand.