That's very true beosboxboy

Those are perfect examples. Some things get messed up because of new animations presented by an expansion pack, or any new info also.
It also turns out that these maxis cars replace some of the original nightlife cars' files. These new cars from maxis, for example, have different light glows and replaces all original NL cars and custom cars lights. So I wouldn't be surprised if they replace other things with cars too. So both types of cars are linked in a way and borrow files.