Why is Chang afraid to operate or give medical advice on Grey's Anatomy?
because in the season finale last year she had a gun held to her
head while she was operating on Dr. Shepherd who was shot. The gun
man told her to stop and let him die while Meredith (his wife and
her best friend) was standing right there, so she unhooked the
heart monitor while he wasn't looking and when the gun man left the
room she continued to operate. Derek lived but now she is
traumatized to operate because the gun man shot his gun a couple of
times and then held it to her head and forced her to stop.