Another great Windows XP feature with another great set of problems: Offline Files. If you have a laptop or unreliable inter-site connectivity, then you know of the necessity of keeping a local copy of your shared files to make them available at all times. The Offline Files feature automatically keeps track of which files need to be synchronized for you, making that offline experience very slick.
Try to do this in a multi-user environment, however, and it will blow up spectacularly. The most common symptoms appear when double clicking a document icon in offline mode. Windows loads the program associated with that type of document, and that program instantly crashes or throws a file error. This happens any time more than one user tries to use the same file offline on the same computer.