Well I haven't experienced it personally but it did happen with my friend and we had to turn the cake around because part of the back had collapsed. She was not prepared for it and I think that is why it happened. On the other hand I believe that the venue that will be hosting the event will have some experience in ensuring that this doesn't happen. I bet it will be just fine! You can always ask them how they deal with such a situation, i.e., how they prevent it from happening.