This is an odd behavior in Office that I have a user experiencing and I've experienced it before but never as much as this user. My history with this has been when clicking an email in Outlook and then you click another it selects all of the them in between. Kind of like you are holding the SHIFT key down. After either rebooting the computer or making sure the SHIFT key isn't stuck it works fine again.
My user on the other hand is having this issue in all Office applications. If she goes to highlight a paragraph in Word and after she lets go of the mouse button, then Word is still acting like you are highlighting. If no mouse button clicked I can highlight the whole page. It doesn't do it every time, but probably 1 out of every 3 when I was testing.
We've rebooted the machine, replaced the mouse and it is still happening. It is a laptop and one of my thoughts is that the docking station might be the issue, but I just wanted to throw this question out there and see what you guys had to say. If you need any more detail, let me know.