This customer came back to me with an issue they were experiencing. When someone tried to save an Office document of a certain contenttype into a document library, they always got a messagebox saying: "This file cannot be saved because some properties are missing or invalid.". That's impossible, SharePoint must be wrong, since the only required fields are, in fact, filled out.

There were some additional fields, but they were set hidden, so that couldn't be the cause. Or could it?

Solution

I remember that some of those, now hidden, fields used to be required. So I set them to optional instead of hidden and sure enough, opening a document in Office revealed that one of them was still a required field. Funny, I just set it to optional... After waiting a bit it automagically seemed to have fixed itself. So my guess is that you have to wait a bit until the contenttype change is "replicated", if you will, to all the libraries. After that, you can set the fields back to hidden and will be just peachy...