Occasionally when my MBP dims it's screen after being left inactive for too long, I'll go back to it and wake it up. I'll usually do this by shaking my finger on the trackpad or clicking the Escape key. However, immediately after entering in my password to unlock, it'll dim the screen again and go black. I then do the aforementioned process of waking it (usually takes slightly longer than the first unlock), and it'll be fine again. It only happens every so often and therefore I can only assume it's something I'm accidentally clicking. Can anyone guess what's happening?