Day the Earth Stood Still, The - Blood on Wrong Side
This occurs in the beginning of the movie after Reeves is shot. He is looking at his shirt in the mirror and he can see that it's bleeding through on the right side, however when he's applying medicine to the wound a few shots further, the bullet hole is on his left side.
When you see the blood on the shirt I'm pretty sure you're looking at the mirror image so it looks like the right side but it's actually the left side.
After watching the movie again I find that I am correct. You can see Klatu from behind standing in front of the mirror and you can see him pull the left side of his jacket open with his left arm. The reflection makes it seem like it's the right side. There is no slip-up!