I haven't seen this yet, but I did have an issue the other day. I charged my batter to full and left it on the charger for about 15 more minutes just while I got dressed and stuff, and then took the phantom outside. While I was airborne at around 200 ft and putting in my waypoints the battery went from 96% to 40% in about 3 minutes, and as I was trying to recall and land the thing, the battery dropped to 16%! I was able to land safely, and recharged the battery and haven't been able to re-create the problem. I. Sure I had the battery seated properly and it definitely was showing 4 green LEDs on the battery...
Weird.