Reproduced with ODF courses:

• Create a validator to easily generate warnings on content modification (only server side warnings)
• Add this custom validator to an externalizable course's attribute
• Modify a course
• Add a value that will generate a warning according to your validator
• Save
• There is a dialog box with the warning => OK
• Click on no button to not save your modifications
• The field in form is not marked with a visual warning => KO