You should be tolerant of other's mistakes.
Andy is mentoring Susan as a scientist, and Susan has messed up an equation.
Andy wants to correct Susan's mistake.
Andy points out Susan's mistake and tells her it's no big deal and shows her how to fix it.
Andy tells Susan he's never seen such a basic mistake done before, and that it's embarrassing.
Susan learns from her mistake and becomes a better student.
Susan loses confidence in her abilities and ends up quitting the program.