If you number all your identities from 1 then there will be duplicated values in different tables. If an application developer gets hold of the wrong integer key and inserts it as, say, a foreign key value, then there is a high likelihood that database won't complain. We all make mistakes. Have positive experience of very appreciative developers from the few times I have used GUIDs, always with replication.
Hmm. While that's possible I suppose I've never encountered it before.
Live and learn, I guess. 🙂