One real-world scenario where it seems to be a problem is for you to show off your work :)
Clicking "create form" (to see the example you provided) returns the identical form error again. I have to modify it to see how it works, rather than using the example you provided.
Not a show stopper at all, but not intuitive for an example website as there is no indication that I need to modify your example form before being able to see the example of what is produced.
(creating the identical form twice is not allowed; not really a problem in real world scenarios)