How can I display an error message in a Page Layout, not in Visualforce?
[SalesForce] display an error message on Page Layouts
Related Solutions
You can display error messages using the addError method on sobject or its fields.
Salesforce docs on the sobject instance addError method: https://developer.salesforce.com/docs/atlas.en-us.198.0.apexcode.meta/apexcode/apex_methods_system_sobject.htm#apex_System_SObject_addError
More info on exception handling: http://wiki.developerforce.com/page/An_Introduction_to_Exception_Handling
http://www.sfdcpoint.com/salesforce/show-error-message-visualforce-page/
I hope this will help you for sure
Best Answer
If you want to do it through triggers: - https://help.salesforce.com/apex/HTViewSolution?id=000176551&language=en_US If you want to do it through validation rules - http://www.salesforce.com/us/developer/docs/workbook/Content/validation_2.htm