In this example the protobuf plugin included in the project prevents a WP8 build if there a protobuf plugin for iOS is also included. Removing it allows the project to build normally.

To reproduce:
1. Open attached project
2. Build for WP8 - an error is thrown: Assets/TestScript.cs.cs(3,7): error CS0246: The type or namespace name `ProtoBuf' could not be found. Are you missing a using directive or an assembly reference?

Removing the plugin from the iOS folder allows building normally (after a restart).

The workaround is to name the plugins differently, so they do not clash with each other.