I came across some funny behaviour while using word 2003 in combination with Sharepoint 2007 document libraries.
When they open a document everything goes well. They can make changes but ,….. when they try to save the changes, the source of the openend document is lost and the editor tries to save the file locally.
Looks like office opens the document read-only.
After some searching i came across the following microsoft article.
In my case, it solved the problem.