Example of odd UIAppearance proxy behaviour in iOS 8.1. When implemented this way, the labels used in controls which should be outside the scope of your app, like the iOS share sheet, are also modified.

gistfile1.swift

// Extend UILabel with a method which sets font, to be used by UIAppearance
extension UILabel
{
    func setFontName(fontName:String)
    {
        self.font = UIFont(name: fontName, size: self.font.pointSize)
    }
}

// Somewhere in the code, call the extension method on the UIAppearance proxy object of UILabel, example:
//
// UILabel.appearance().setFontName("HandelGothicBT-Regular")
//