I saw a taxi floating in the air (4 meter high) doing nothing but engine noise. This happened to me twice. I manged to take some photos but unfortunelly I dont have them any more... . Other cars were moving under it. This took place right next to Santa Marina beach. I saw a samilar thing while playing Vice City where the car didnt just float, it moved while being 8 meters high and followed its course.
PS: Didnt have in both cases any cheats activated.