Had a client call with this situation – which is frustrating if you don’t know where to look.
You look at your slides in Slide Sorter view, or on a printout, and the first slide is not labeled as #1, but as #0.
To “fix” go to FILE >> PAGE SETUP
In the Page Setup Dialog note the NUMBER SLIDES FROM option. Change the “0” to “1” and all is back to normal.
– Troy @ TLC