I upgraded my IDE to 1.6.8 yesterday and today I realised my keyboard was malfunctioning. Windows10 also updated yesterday so I blamed Windows.
Then I realised that when the Arduino was unplugged, the keyboard was behaving well again.
I downgraded to 1.6.5 and all is good again.
I just wanted to let you know. The "malfunction" consisted in keyboard not responding for 1-2 seconds every now and then (every few seconds) and then repeating the last key which was pressed several times. It would then work fine for a few seconds to then get stuck again.
I tried a different keyboard with no luck. My keyboard and the Arduino are connected to the USB hub which is integrated into my DELL monitor, but it's always worked well. The mouse, also connected to the same hub, worked ok.
I hope this helps fixing the issue.