I recently bought a Canon Canoscan 5200F flatbed/film scanner, which has a number of physical buttons that serve as customisable shortcuts to e.g. scanning PDFs, opening images in Photoshop etc. The buttons, however, only seemed to work for my user account (which I used for installing the Canon software). When other users were logged in or switched to (using fast user switching), the buttons would do nothing.
An inspection of my startup items revealed a program by Canon launched automatically. The path to the program on my machine is /Library -> CFMSupport -> CNQL2406_ButtonManager. After launching this application as another user, the buttons started working for them also. Furthermore, it seems that the buttons always work for the active user, so if you're using fast user switching, multiple instances of the button manager software do not interfere with one another.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050119163920457