Same colleague ... LOVES to correct people, on everything. In fact, he admitted how much he loves it, from pronunciation to, well, pretty much everything anyone says, not even related to work. So, every so often (well, a little more than every), I intentionally mispronounce things. Or, after he's corrected me, continue mispronouncing that word, as many times as I can that day. Not sure that's really a practical joke, but it works for me and my other colleague.