Though it's frustrating to me that Lightning has brought with it custom overlay dialogs that don't recognize the Return key for the default button, I'm grateful Salesforce has generally implemented a Command/Control+S keyboard shortcut for Save. However, I discovered that the save dialog for Change Closed Stage, the button on the end of the standard Opportunity path bar, does not respect this shortcut.

https://success.salesforce.com/ideaView?id=0873A000000lKjwQAE