There are numerous errors that can occur when a user submits a form. They may have failed to complete a "required" field or they may have entered data in the wrong format (e.g. a date field entered as MM/DD/YY instead of DD/MM/YY). They may have attempted to purchase something that is no longer available or they may not have selected what they want to purchase! Regardless of the error, it is important to provide clear feedback as to what the problem is so that the user can make adjustments and not repeat the error.
Once a form has been submitted and error(s) have been detected, provide a clear error message that conveys this to the user. The more specific the error message, the easier it will be for the user to correct it. For example, "Error: Q6 has not been completed" is better than "There are errors with this form". Try to ensure that the other information entered by the user is retained so that the user does not have to complete the entire form again.
No example is available.