I can be a real prick sometimes

What makes me a prick this time is not so much what I did, but that I so thoroughly enjoyed what I did. At work there's this guy who seems to know what he's doing, but is either constantly super busy or lazy, so he tends to half-ass a lot of the work he's supposed to be doing for us as high priority. This has ground on just about everyone's nerves quite a bit, and while he's gotten shit for it, I've never had the pleasure. So I found some things that were missing between our work system and a sheet he sent us. He then replied that he couldn't find what I was talking about (meaning he didn't look very hard). After making sure with my manager I wasn't being too much of a dickhead, I happily (I was literally giggling half the time I was doing this) sent back a document with a screen shot from the sheet he sent us and a screen shot from the system that had the missing bit pointed out with a GIGANTIC RED ARROW that there's no way he didn't notice. As a way of example, this is essentially what he saw:

           Minor detail one
-------> Minor detail two

Honestly what I did was actually helpful in that it did point out what he was missing and it started a useful dialogue that found other errors as well, but taking such pleasure at getting to use such a very sarcastic response while disguising it with good intentions was just too much fun for me. Originally I was going to use mspaint to half-assedly draw in red circles which does have a certain flair of its own, but the giant red arrow feature in SnagIt was way better. The giant red arrows really just made my day. Yay sarcasm!