HI Johann,
I have seen the same issue. In those cases I put a button on the form that says something like 'Press to Validate your entry'. That way, they are told to do something. SO, if they complain that "they put in a value and it didn't work" then you can ask them if they pressed the "Validate button" - and it becomes your chance to educate them!
I am not sure if there is a $Script command that you can trigger before the form is closed but then it may not have the value they entered because they did not acknowledge it with a Tab or Enter.
Ron