I have a strange problem when I press up left arrows and space simultaneously. Why doesn't the space work? I tried to test here http://jsfiddle.net/vor0nwe/mkHsU/ and there is the same problem, you can try! if you press up , left arrows and ctrl it works! With left +up + g no! This happens in Firefox, Chrome etc... why?
Most likely the fault is of the keyboard (see correct answer)! I Tried to press left (hold) and right shift (hold) and x in a text document. x doesn't appear. http://xahlee.blogspot.it/2010/06/keyboard-key-ghosting.html. My keyboard is logitech 200 :-(
This is actually probably your keyboard, it's a phenomenon called "Key Ghosting", certain key combinations wont work on some keyboards.