I've had that kind of thing happen too. But I think I've only had it happen about 3-4 times total since I started using iOS 7 back in the beta days about a year ago, so it certainly hasn't been something that was of too much concern (although a curiosity as to what might be behind it). A reboot solves it, as you said, and I've seen some others post about it with pretty much the same information, although in a case like this it's rather odd that it's happening so often.I go to use phone and sometimes it goes straight to passcode screen which is a different colour to my usual one and buttons are frozen, other times I get the slide to unlock screen which is also a different colour to my usual screen and it doesn't respond. I have to reboot phone. Happens randomly, sometimes once a day, sometimes I can go 3 days before it freezes again.