Building using Xamarin.iOS and Visual Studio 2012 stopped working for me when my debugging target is a physical device. Clicking Start resulted in the message:
System.InvalidOperationException: The operation cannot be completed because a build is already in progress.
It worked fine at invoking the simulator.
This appears to be a bug, but there is a workaround: Go to the solution's Configuration Manager and uncheck Deploy.
This issue and workaround are present in Xamarin.iOS 1.8.365.