People unclear on the concept of using an office scheduling tool.
My company uses Microsoft Outlook. Yes, it has its problems, but the calendar/scheduler works pretty well. To set up a meeting, you find the people you want to invite in your address book, and all the schedules display stacked one on top of another, so you can easily find available times. However, this only works if people actually put their meetings, vacations, out of office times, etc, in the calendar. It's infuriating to schedule a meeting in a nice, open slot, only to have one or more attendees reply with, 'Oh, sorry...I have a conflict/I'm out of the office that day'.
This behavior leads to the less annoying but still mildly irksome habit some people have of asking you, 'When are you free for a meeting?' Look at my doggone schedule! That will show you exactly when I'm free, because I actually use the scheduler for its intended purpose.