If you’re in the middle of dragging a file or other object, you can press the Esc key to cancel the drag without having to remember where you dragged from. After you press Esc, you’ll see an animation of the dragged icon jumping back to its original location. This is especially useful when the original window location is out of view, which can happen due to the autoscrolling and spring-loaded folders that OS X provides when you drag. The reason Esc trick is better than just letting to is that when the original location is out of view, letting go of the mouse button might drop the icon somewhere other than the original location or the intended destination. Esc guarantees that it goes back to where it was.