I had an occasion where I had just started to play back a movie in MediaPlayer, when a call came in. The video stuttered a lot and the device was pretty unresponsive but I got no notification that there was an inbound call. However, I did get notified that there was a missed call afterwards.

This outcome could have been worse - actually taking the call without the spare processing grunt to handle it well would have been a bad experience. It does seem though that better resource management wouldn't go amiss.