It's even more infuriating when it's a section I've zoned myself. Luckily I don't have to do that terribly often. Even so, when I'm shelving I'm pretty gods damned obsessive-compulsive about having stuff in some semblance of order, or at least pulling books that obviously don't belong, and this shit seriously slows me down pretty badly. I've even been told not to worry too much about the order, but when there's no semblance alphabetical order to be found that's pretty much impossible. It's worth noting as well that there are actually some sections that are always so bad that we've just completely given up trying to make them alphabetical at all. Crafts & Hobbies is one of these. A long time ago we actually tried to maintain alphabetical order, but we're so understaffed these days that it's pretty much impossible.
That reminds me of another pet peeve: Books that don't have the authors on the cover or the spine. What the hell are the publishers of these things thinking? Christian publishers are particularly bad with this shit, I've noticed; I find this example particularly infuriating since it's one of the sections we actually try to keep in order, but about one in thirty books in that section doesn't list an author. It's completely counter-intuitive. If you want people to buy your book, they have to be able to find it. A lot of the time the author is something you wouldn't even be able to guess. This crap wreaks havoc on alphabetical order because even customers who want to keep things in a decent amount of order have no idea where to put it. There are some sections (the art section is a notable offender) where something like half the books don't have authors listed. It drives me up the wall.