Why are my mesh edits reverted? I've received a model from someone else. Any changes I make to the mesh in Edit Mode seem to be undone when I return to Object Mode.

On the left is the mesh in Edit Mode, on the right is the same mesh in Object Mode:
Try going through the following checklist:


Check to see if the problem might be caused by Modifiers.
Try turning off or deleting Shape Keys on the Object Data tab.
Clear out any Keyframes
Clear displacement and rotation of bones
Try removing the armature from the object