I noticed this a couple of weeks ago and just figured out what was happening. Thought I'd share.
I was copying (or moving) objects and dragging them to another part of my drawing when all of a sudden the original object I had selected to copy was replaced by another object that I had passed over while panning. So instead of a line I had a circle.
Sometimes instead of the object "shifting" to another object a group of selected objects wouldn't move/copy at all, instead only one of the selection group would move/copy.
After a week I figured out that it was the Drawing Inspector that was causing this problem. Turned it off and everything worked fine, don't ask me why.