When the little boy and girl are in the car looking at the goat(to their right), the goat is on flat land. The T-Rex starts to push them to that side and that side becomes a cliff!
This is not a slip up. Yes the cliff does appear magically but it was done on purpose. in an interview with the director he says that he did realize this but didn’t care because it added to his illusion of the movie. So it was don’t on purpose and not accidentally so this is no slip up.
Well, I agree with you when you say that the magic clift it's not a "slip up" because the director noticed it but he didn't corrected it. But I thing this point should be discussed cause it really confuses the audience. No matter how lost you are in the movie, it's really disturbing to see how the ground suddenly dissapear.