My spiffy new Logitech mouse is fantastic apart from one annoyance: there's a "Back" button beside my thumb that I have a frequent habit of accidentally pressing. Usually it does nothing but sometimes it skips back a song in iTunes, jumps back a page in Firefox or something random in other programs. Fortunately I can disable it and that's exactly what I've just done!
Mice should only have three buttons and a scroll wheel. Additional buttons are just silly.