On Sunday, I walked into my office, sat down at my computer, and grabbed a hold of my mouse. Suddenly, a loud electric shock came out of my hand.
I started using the computer, and before long, I noticed one of the two thumb buttons wasn't working. I ran Counter-Strike Source and noticed that my flashlight was on when I first started playing the game (the key I used to turn on the flashlight happened to be my second mouse button). Then, I started VirtualBox and noticed that I couldn't click on any buttons or menus, although I could X out of the program.
I tried a different USB port and a different mouse, but nothing seemed to fix that problem. Then I remote desktoped into the system, and everything was working. Strange.
The next thing I tried was I completely uninstalled the drivers for my mouse, and then plugged in another mouse, and rebooted the computer. Everything worked.
I plugged my old mouse back in, and the problems came back. So I guess that was it - my beloved Logitech MX518 now has Mouse4 being pressed all the time, even when I'm not pressing it. So now I'm stuck with a cheap HP 2-button mouse, at least for the time being. (Why couldn't I have killed that one instead?)