I just had the same problem; I received a phone call that I needed to respond to with key commands, and the Galaxy S5 was shorting out, the screen would go black, the keys would not respond, etc., and I ended up missing the chance to place the key commands in the very important call. Needless to say I will be returning this S5 to the store in a matter of days if it has bugs like this. Otherwise the phone is awesome, so it's really a shame Samsung couldn't iron out simple bugs in their supposedly brilliant smartphone before marketing it. An iPhone would not have such a problem. If anyone has any ideas on how to fix this problem, please post something so I don't have to waste my time switching over to an iPhone again.