OS X's Hot Corners feature is a way to add quick access to some Mission Control features, but if you drag your mouse pointer into one of your Hot Corners by accident, you may inadvertently activate that feature when you didn't want to. Adding a modifier key to your Hot Corners can easily prevent that accidental activation. Here's how.
Open System Preferences on your Mac, and then click the Mission Control preference pane.
OSX's System Preferences
Next, click 'Hot Corners...'
The Mission Control Preference Pane
Choose the corner you want to make 'Hot.' In the list, you'll see a handful of options that can be super useful to have quick access to. Activate one, and dragging your mouse to that corner of your screen will quickly activate that feature.
Hot Corners selection menu
To add the modifier key, simply hold down the keyboard key that you would like to associate with the Hot Corner and make your action selection. This works with the Shift, Control, Option or Command keys.
Hot Corners selection menu with a modifier key
I’m a big fan of using this setup to put my Mac’s display to sleep. My home office is in the bedroom, and as much as I enjoy my photo screensaver, when it’s time to hit the sack, I want that screen to be dark.