Double press on power button sometimes doesn't display lockscreen
The feature to be able to double press on the power button and instantly get the lock screen to enter your code doesn't work 100%. Sometimes after a double press the phone responds as if you've only pressed the button once and shows you the screen with the clock and so on and not the screen with the option to enter your code.
I've not been able to figure out what causes this behavior but it usually happens when the device hasn't been used for some time, say, for an hour or more.