If you watch
http://www.dealnews.com and the like, you'll find a lot of these. Dells system is so automated that sometimes the order will have already been processed before they have a chance to stop it, and it's cheaper to ship you the item than it is to cancel it.
They screwed up last month with the newest Creative Nomad MP3 player, listing it for $79 instead of $379 or something... 4 guys from the office ordered one, none of them showed up.