I’m currently working on a project in Blender with a team in class. Problem is that my computer is newer than the others and can run 2.64 while the others run 2.61. Now, I wouldn’t think this a problem at all. I mean, a .3 version shouldn’t make a huge difference, should it? Well, I would think so, but whenever I move objects from .64 to .61 I lose the faces, and whenever I move projects from .61 to .64 the key-frames in the animation change. I was able to re-fill the faces mostly fortunately, though one object is a horse that has been extruded from a simple box and cannot be filled due to how complicated it is. The key-frames also won’t fix and must be re-applied if I ever wanted to run it on my computer. Luckily, our project is not broken as long as we stick to .61 though I obviously spent a lot of time on the horse and would be greatly disappointed if I were not able to use it; so any help is greatly appreciated. 
The mesh format was changed in between these versions. When you save from 2.64 enable the Legacy Mesh tick box in the save browser windows. Just stick with using one version in a project. You can have different versions installed without causing any clashes