Was fighting an annoying aspx / R# error for a couple of hours this morning. I’ve reported this as a bug to R#:
http://www.jetbrains.net/jira/browse/RSRP-96241
The basic problem: R# doesn’t highlight correctly if your Inherits directive is not fully qualified. Even if it is in an @Import directive or in your Web.config. The solution is to always fully qualify your Inherits page directive. Or for R# to recommend importing it or fully qualifying it or actually reading the web.config correctly. Go vote my issue up. :)