If you copy an object (ie: ctrl+C), or select an area and copy all objects inside that area, and then paste directly again (ie: ctrl+V), the newly created objects are actually placed UNDER the original object.  

So, when you try to move the newly created object, you end up moving your original. 

This gets particularly frustrating if you copy/paste a big bunch of objects at once, and then do something else before realising, and can't access the undo function any more.