I was trying to navigate through one of my sets today because I thought one strip needed editing and when I tried to use the back button, I would keep getting "Comic not in set" pages like this one:
http://www.stripcreator.com/comics/choadwarrior/sets/vacation/2
You can see, from the url, that instead of a comic # at the end, it's just a "2." The same thing happened to another set of mine where i got an "N" at the end.
The forward button is working fine. I'm using firefox.