When the nurse mounts the packet of Bart's blood (appropriately labeled, ''HUMAN BLOOD'') on the stand, it is about four-fifths full. When Smithers says, ''Go ahead'', the packet is completely full.
When Smithers is giving Larry his instructions on beating Homer, his (Smithers') suspenders vanish from the front of his body when he bawls, ''He saved Mr. Burns' life!''.
When Moe refills Homer's beer, the glass disappears when Moe hands it to Homer, then reappears in Homer's hand.
Smithers couldn't have given Burns his kidney if his blood type is different.
Instead of pretending to be Mr. Burns, Homer could've told the man at the post office his real name and that he has to get back the letter he "mailed by mistake" since his name is on the return address.
There is no such blood type as Double O Negative.