ERROR: Only Content controls are allowed directly in a content page that contains Content controls.

By default, SharePoint doesn't seem willing to share some of its internal errors with developers opting instead to use a generic "Unknown Error" page. The informative (and I use the term loosely) error string above wasn't displayed in the browser until the file C:\Inetpub\wwwroot\wss\VirtualDirectories\80\web.config was modified to disable custom error messages.

By changing <SafeMode CallStack="false" ...> to <SafeMode CallStack="true" ...> and changing <customErrors mode="On" /> to <customErrors mode="Off" />, the generic error page is replaced with a more detailed error message and maybe even a callstack.

As for the error above? It was caused by a comment that had been added to an .aspx file. For some reasons, comments were not allowed because they didn't qualify as "content controls".