Another Possibility
Sorry for not responding to the original challenge but in my opinion the most l;ikely problem is that the user performed the following steps:
1) Set up the introduction
2) Deleted the first slide in the actual presentation (the destination from the loop)
3) Added another slide in the introductory loop.
After step 2 it will still work correctly, clicking on the action button goes to the new start of the actual presentation but at least in PP2007 it seems to change to a reference to the nth slide rather than the actual.
In my test I set up a presentation with a 3 slide introduction and 3 slides in the presentation. The action button jumped to the 4th slide. When I deleted slide 4 the action button still jumped to the 4th slide. However when I now added a slide in the introduction the action button still jumped to the 4th slide which was now part of the introduction.
