When headphones are plugged-in and the proximity sensor is covered (for ex. the phone is carried in an armband) the screen can not be turned on during a call. This prevents you ending the call without first taking the phone out of the armband.
This bug occures on my SGS I-9000 it is a port from NexusS on miui.us. I don't know if it also exists on other devices. When having headphones connected and you have your phone on an arm band and you answer a call, there is no way to go to the call screen. The screen is turned off immediately...