I was charging my phone and was looking in the notification bar. It just locked up. I tried to press the home button and then the power button, but it was unresponsive. I unplugged the charge cable, but the charging light stayed on. I tried to hold down the power button for a few seconds, and it didn't make a difference. I finally did a battery pull and now the phone works fine.
Mine locked up on the task manager twice. Both times I was tapping the "End All" button like a mad man just to see how high my free space could go.
I learned my lesson on rapidly cancelling all tasks at once, 15 button taps in a row. (Don't do it.)