Revealing mistake: When George is on the bridge and says "I wanna live again", and Burt drives up, in the shot where George hugs Burt, then Burt pats him on the back, you can see Burt's handprint in the fake snow that was used (a mixture of foamite soap and water) on the back of George's coat. (02:02:35)
Revealing mistake: When Tom is sitting by the window looking at Toodles at the start of the cartoon, his tail is missing.
Revealing mistake: When Sylvester is on the run from Porky, he dives under the bed, but the cels of him diving were placed over the bed, so that he appears to dive on top of the bed and disappear.