Mouse sensitivity randomly changes

Sometimes, without changing any settings in my mouse settings, my mouse will just get more or less sensitive--all of a sudden, moving the mouse moves the cursor a lot faster than it did before. I have no idea why it's happening or how to make it stop. It doesn't happen very often. Maybe once a week or so. Sometimes it's noticeable when I switch user accounts, but other times, it changes sensitivity without making any changes to the system at all.

If it matters, the mouse is a wired Logitech MX518 plugged into my Apple keyboard.