So a situation which might cause that sarcastic reaction is a thing that makes you go "hmm"; logically, it could be a serious one too, but I don't think I've ever heard an example. The phrase was popularized in that sarcastic sense by Arsenio Hall, who often uses it on https://youtu.be/T0zWXvhEQwo?si=wNc6ihu97ShtIl7u