This is a good one
In Xamarin Studio in order to test an async method the test method return type has to be void :
[Test] public async void GetCarWashesAsync_ShouldReturnCarWashes ()
In order for the exact same test to work in Visual Studio the return type has to be Task
[Test] public async Task GetCarWashesAsync_ShouldReturnCarWashes ()
If in Xamarin Studio the return type is Task the test just gets skipped (project builds but test does not run) and vice-versa.
Full example here :