What are some common mistakes students make with R and S configurations?
Failing to use molecular models.
It is always a problem to represent a 3D geometric figure on the page as a 2D picture. The use of models is always advized.
When we draw a chiral centre (or stereogenic centre) on paper, 2 bonds are presumed to lie on the plane of the paper, 1 bond out of the plane, and 1 bond into the plane. It takes some practice to represent this unambiguously. Here is a tip in this regard. Suppose you have represented your chiral centre correctly on paper. The interchange of any 2 bonds gives rise to the enantiomer. Interchange again, and you get the enantiomer of the enantiomer, in other words the original species (i.e. the mirror image of the mirror image is the original!).