When you place a map in a item frame, leave the world, then come back, the map has the undiscovered lines on it. The only way to fix this is to either wait 5-10 minutes or break the item frame and place it back. This bug is quite annoying... Please fix it soon.
I'm playing in 1.6.2, and when a player joins, the map doesn't load in the item frame. A way to fix this is to hold the map in your hand. This is an annoying bug, because all of the maps on a wall are blank until reset manually and individually.