Mrs. Grubman - S2-E4
Revealing mistake: Mrs. Grubman is lying in the coffin. She noticeably swallows when she is supposed to be dead.
Revealing mistake: As Dr. McNamara is marking Megan O'Hara for her breast reconstruction surgery you can see the lower line of her prosthetic "removed breast" from her mastectomy on her left side. It is not the surgical scar from removing the breast, that is visible about an two inches higher closer to where the nipple should be. My aunt had a mastectomy, I know there should be no scar here.
Revealing mistake: (Across Season 4) During several close-ups of baby Conor McNamara over various episodes in Season 4, you can see the off-colored slip-on "gloves" that are used to represent Conor's deformed hands.
Answer: It's a computer graphic.