Revealing mistake: While Matt is lying in his hospital bed, he hangs up on a call with Emma. You can see his left foot moving. He's not supposed to be able to feel or move his lower body.
Revealing mistake: There are bullets placed on the counter, the one casing has a hole in it from being drilled to show it's a dummy round and there's no gunpowder in it. (00:14:45 - 00:15:20)
Revealing mistake: The bullets the priest takes out of the ammunition tray have no primers. (01:01:33)
In Sickness and in Health - S4-E18
Revealing mistake: The baby is obviously a mannequin or doll when it's underwater.