Extruded Part disappears after switching from Edit Mode into Object Mode As the title says: my newly added parts in my mesh disappear when I change from edit into object mode. 

And if I change the position of my mesh in edit mode it's still at the same spot in object mode.

My mesh is rigged and weight painted, I also made a few animations.

I already checked my layers, tried Alt+H but nothing helped. Making a new mesh wouldn't be a big deal, but I can't make always a new mesh if get the same problem, so I want to know how to solve this problem.
Shape Keys

I just deleted the Shape Keys for Head_Cube.005 and now the extruded arm face stays throughout the animation:  



I don't know any more about Shape Keys, but here is a link to them on the Blender wiki:
http://wiki.blender.org/index.php/Special:Search?search=shape+keys&amp;fulltext=

If you need the shape keys, then you will need to figure out how to edit them to incorporate the extrusion throughout the animation. I'd help more with them if I could, but I know nothing else about them.