I've hit "Show preview" many times and wondered why my changes weren't applied only to realize I was editing /sandbox and the page was using the live version, or something to that effect.
Previewing a page that does not transclude the template you're editing should give you a warning instead of showing you the completely unaltered version of the page.
Likewise, the autocomplete function of the "Page title" textbox should only suggest pages that transclude the template you're editing.
(image borrowed from T279021)