I've recently come into a 2012 15" MacBook Pro with some water damage. The only issues I've seen so far after a fresh install of Sierra is that it boots into Safe Mode if I don't hold down option, broken left shift, and an interesting phenomenon I can't find anywhere else with the left command key. I've been using the computer for a few days with no problems except when I press the left command key the system acts as if shift is being held down and will not stop until I restart. So that means windows open and close in slow motion and I can't really use it until I restart. I'm at a loss as to a solution, has anyone else had this problem or know a fix?
You need to replace the keyboard. No other fix for that issue as the water has caused corrosion within the keyboard circuitry. It is a time consuming repair but isn't that hard if you have a good guide like on fixit.com and are reasonably experienced at fixing computers.