When this has happened to me as boss, I always take the side of my staff member, no matter what they have done. It usually doesnt make me popular with the other person (or their boss when they get involved) but that doesn't bother me - all part of the fun. Tends to get a good working relationship with your staff though. A quiet word after is all that is needed.
I'm both the boss and the person doing it at times; some people need a kick up the jacksy and if this is the only way to get them to do it then I'm all for it. I work in an area that often requires responses pretty quickly and if you're being slack or difficult to deal with then you'll either get an email with your boss copied in or me on the phone; me on the phone is the least preferable of the options I am reliably informed.