There was this thread about shoddy bodywork, and how a shop can repossess your car if you don't pay up, even if the work was not performed. Even a well reputed shop can change owners and run into financial problems.
http://forums.pelicanparts.com/porsche-marketplace-discussion/974583-53k-body-work-73k-body-work-you-decide.html
I'm not sure of the best way to avoid this last one, or at least minimize the risk of having your car taken hostage at a scammy shop. For me, "small job at small shop, and big job at big shop" was my conclusion. I will never get body work done at a small shop.