Upon loading a page in the designer, double clicking on a form field and selecting all the contents of a text box will not properly allow you to edit the contents of said text box. See here:
Notice the discrepancy that occurs when I selected everything in the Placeholder text box and type "firstname.lastname@example.org". It does not replace the highlighted text, but rather appends it to the end. It displays appropriately in the text box, thus creating a confusing user experience.