Continuity mistake: In the animated sequence, just before the kids destroy Sister Assumpta's office, Skeleton Boy's cast remnants switches from left arm to right arm and back to left arm. The left arm is the correct side as Tim (who is Skeleton Boy) had his cast on his left arm. (01:20:00)
Suggested correction: Your observation is correct, but this isn't necessarily a "continuity" error. The nature of comics or cartoons lends itself to imagination, creativity, humor, etc. Drawing comics is less tedious these days, thanks to computer programs and "cut and paste" techniques; the chances of a cast accidentally placed on the wrong arm are slim to none. Skeleton Man's "drawing" has many inconsistencies - number of ribs, teeth, finger bones, EYEBALLS or dark holes - with comics, it doesn't really matter.
Continuity mistake: Throughout the scene where Roland and Maud are sitting on the floor looking through the things in the box from Ash's grave, the head of Maud's hairclip switches constantly from one side of her head to the other.