Anone notice how the black convertible kept changing?

Originally in the beginning when Sara drives Alex home from the bar she used to work at, it is a Toyota MR2 and then when Sara and Eric drive drunk Alex home from the party, it is a BMW Z3. After Sara tells Brooke off from getting her scholarship revoked, it is a MR2 again in the moving shot and when they get out of the car it is a Z3 again. Sloppy continuity errors.

Another thing, if Alex is so rich that she has this giant house to herself, why is she driving a crappy 10 year old plus convertible? The Toyota is from the early 2000's and the Z3 that it morphs into is from the late 90's.