"Click-through" is a term for being able to interact with buttons and controls on a background application without clicking first to activate it.
Click-through is normally disabled in OS X. There are some exceptions where the first click is active, like iTunes player buttons, Safari UI, and Chrome UI (but not active webpage content area). John Gruber (Daring Fireball) has blogged about click-through quite a bit. Apple says:
In general, you want to allow click-through for nondestructive actions that users might want to perform when they’re focused on a task in a different window.
The Command+Click shortcut enables click-through without activating background apps.
So I noticed a little problem in OS X. In the typical situation where click-through is disabled, a double-click actually fires a double-click in the background application. I find this behavior to be inconsistent and annoying because, wait a minute, isn't the first click only supposed to activate the application? It's causing the kind of unintentional behaviors that blocking click-through was supposed to prevent in the first place.
One example is in the case of backgrounded Spotify. Single-click on the Next Track button, and the app will take focus without activating the button. No track advance. But, if you double-click the button - as you would if you expected the first click only to take focus - the button is pressed twice, advancing two tracks instead of one.
I believe there are two problems here; first, the Spotify buttons should be enabled for click-through, as the iTunes playback buttons are, but also, OS X should guard double-clicks from falling through to backgrounded applications that are protected from click-through. In effect, passing a double-click where click-through is disabled prevents a user from sending a quick single-click to the background app.