Web applications without bugs - it is a myth.
Every developer has many problems when he should reproduce a customer's bug. Usually, the customer cannot provide enough information about conditions before an exception (field values he entered; page which was opened before an exception, etc.).
It will be very useful if the application can produce (restore) the source of the opened web page with the data entered, as it was before an exception and store it (e.g. into a database). In this case, the developer can get the source of the page (e.g. from the database), put it to an HTML file and open it in a browser. The developer will get a copy of the page as it was before the exception.