Masking–selecting parts of an image to isolate changes–is a constant part of any Photoshop work. We select with the Marquee Tool, the Magic Wand Tool, quick mask mode, the Select Color Range command, and many, many other tools, commands, and methods. CMD+D/CTRL+D is the typical way of deselecting something or removing a mask, and it becomes to most Photoshoppers one of the most commonly used commands; it becomes an automatic movement, even. So familiar is that shortcut, in fact, that it’s once in a while used too soon.
If you unintentionally deselect a complicated mask or realize after working on the image that you need the selection mask back again, do you have to painstakingly redraw it all over again? Nope.
On the Select menu is a humble little command called Reselect. Even if you haven’t saved the selection mask to a channel, Reselet remembers the last mask you used and deselected. Call this command and your mask will return instantly.