When you edit the content of a page in the VisualEditor, and then try to leave the page (e. g. due to accidentally pressing the wrong button), you get a warning (the message varies by browser, I believe):

This page is asking you to confirm that you want to leave - data you have entered may not be saved.

If you’re editing TemplateData, and hadn’t made any other edits to the wikitext, then there’s no such warning, and you may lose all your work registering parameters at the slightest misclick.