have someone walk up behind you and tap on your desk I've got two colleagues that are horrible about this. Fortunate enough to have an office with a door, and

have someone walk up behind you and tap on your desk I've got two colleagues that are horrible about this. Fortunate enough to have an office with a door, and play music quietly at my desk so I'll hear it if someone knocks--since I face slightly away from the door. Except they'll skip knocking and just walk in and around my desk to do the shoulder tap if the door's open. I've been training them out of this behavior by saying "oh hey did you knock? I must have missed it, what's up?" Time will tell how this goes, heh.