It happened to me too on a generated level. I don't care too much about that bug because I can always exit the battlefield and re-enter.
It is also usually easier than that because usually only one character goes out. If this is the case, you can exit the floor and re-enter with the other and solve the problem.
At least, someone isn't writing a faq on going out of bounds on a game. I think they did on Daggerfall of course that was 3d collision detection and this is essentially 2d collision detection so Daggerfall had a little harder problem. As long as you saved, falling through stairs wasn't too bad of a problem. The big problem was the almost impossible generated quests in Daggerfall.
