The world I started in 14w27b crashed 3 or 4 times in a row when I played it in 14w28a, probably due to ladders. After it crashed, it disappeared from my worlds list, but it's still sitting in "Saves".
I started a new world in 14w28a yesterday, and today after planting pumpkins, it crashed 3 or 4 times, and disappeared from my worlds list. It is also still in "saves".
Most of my crash reports should be sent to you.