If you get this error, and haven’t touched a page layout, don’t worry:

This page is not using a valid page layout. To correct the problem, edit page settings and select a valid page layout.

I panicked at first thinking about how I might have destroyed my development environment, but it turned out to be something very simple.

During a quick prototype, I used a querystring parameter of “ID” in a web part. It had the unintended consequence of throwing this error. Simply changing the parameter name to something else will fix it up quick!