I just updated with a clean install yesterday to 1.5. I have had this issue happen to me twice since I've installed it.
Phone is asleep, and someone calls. Phone turns on to show sliders, but it's the unlock/mute sliders, not the answer/decline sliders. Also, my background, which you can normally see at the lock screen, isn't there, and neither is the name of the person calling. It's a generic lock screen.
Nothing I do answers the phone. It sits there and rings, and then goes to voice mail. However, even after the call is finished, it continues to vibrate. Only way I can get it to stop is to pull the battery and reboot.
I have tried calling the phone while it's asleep myself to see if it happens all the time, and it doesn't, it's intermittent.
I am running stock Liberty 1.5 with no custom theming or overclocking etc.
Apps that may be pertinent:
Adobe Flash Player 10.1
Chrome to Phone
wp clock (background clock)
Also, the last weird symptom which may be unrelated but i'll include it anyway, is that sometimes when I pull down the status bar, it'll freeze about half an inch down and won't respond. I have to hit the back button, and do it again before the bar will come all the way down.
Not being able to answer call is huge.. Any advice?