Every now and then Visio for Enterprise Architects (and I suspect other versions) seems to to corrupt itself. While this is the version from 2003, it still appears to be the last one with full ERD support. I'll get an error about a relationship not connected at both ends. The only problem is the relationship is not visible on the diagram anywhere so there is no way to "fix" it, or delete it. Luckily I found a solution on Microsoft's technet forums. I'm just repeating here with some additional annotation the advice I found there:
- Select Tools→Macros→Visual Basic Editor from the Visio menus (or hit Alt-F11).
- Select Tools→References from the Visual Basic editor.
- Check Microsoft Visio Database Modeling Engine Type Library and
click the OK button to dismiss the dialog. Yes, the are in alphabetical order, and notice this is under "Microsoft" not under "Visio"
- If you don't have a code window open use the Insert → Module
- Copy this subroutine shown below and paste it into the text area under General.
- Select Run→Run Sub/UserForm (or hit the play button)
If you switch back to your ERD view and run a Model check again the error should be gone.