I was working on another "dummy" cake today, and realized after I had the base fondant on that I had a couple of gaps between the top layer and the bottom (I usually have gaps).
I didn't want to make a batch of royal icing to smooth over the gaps, so I took a 2" ball of fondant and melted it in the microwave for about 10 seconds. then I added a tablespoon or two of HOT water (cold will make it set up fast), and stirred until all lumps were out. then I added powdered sugar to bring it back to the consistency of royal icing. i piped it around the gap and smoothed it with my finger. Voila! success. I'll have to try it with colored fondant next -- should be much easier than trying to match fondant colors?