File or assembly name ‘System.Windows.debug.resources, Version=2.0.5.0, Culture=en-US, PublicKeyToken=7cec85d7bea7798e’, or one of its dependencies, was not found.

This is apparently a known issue in WP7 Mango beta 2 & RC and can make debugging that bit more interesting.

One tip is to look at the Call Stack and ensure that “Show Parameter Values” is ticked (off by default):

Also see this post for some more discussion. It suggests this happens when you are checking for all thrown exceptions (Debug –> Exceptions…).