o reproduce:
1. Create a new project
2. Build for WP8
3. Open up the solution
4. Open up the WMAppManifest.xml
5. In the requirements tab check one of the possible checkboxes (like "ID_REQ_GYROSCOPE")
6. Right click the solution and press "Rebuild"
7. Notice the error in console: "The element 'App' has invalid child element 'Requirements'."

When there is <FunctionalCapabilities> element in the manifest, but no <Requirements>, setting a requirement in UI places <Requirements> element after <FunctionalCapabilities>, which then fails build.

Workaround is to open manifest with text editor and reorder element manually.