Stoo, that's exactly what it was.
The problem was the ad didn't have a type selected and it affected the list view, although not the item view.
That's a bug in the Classifieds app in that it's a required field but still lets you not select one, I'll let the app developer know.
Just for the sake of understanding the troubleshooting side...what code did you look at that you identified the "type" as a potential problem?
Are you looking at the source PHP files on the server or something through the ACP?