Phone favorites is a black screen

[ventmore]

I unlocked the phone as usual, and double-clicked the home button to get to my phone favorites. All I got was a black screen with a title bar at the top, and the phone icons at the bottom. Tapping any of the other icons took me to their respective lists, but tapping to come back to favorites gave me the same black screen.

Switching the phone off and back on solved it, but I've never seen this before, and wondered if anyone else has.

Cheers
Neil