happens with the touch pad and the external microsoft mouse which is wired.
To recreate, I am in webmail, typing a note and I try to use the arrow keys to back up in a line I have typed. The cursor looks like it is still at the end of the line but the arrow key has actually backed up my cursor. I can tell where it is by typing again. The insertion point has moved but the cursor (represented by a bar) is still at the end of the line.