Underdog

When Underdog is re-entering Earth's atmosphere he starts to burn up, when he crashes back to earth and Jack rushes to him and picks him up, you can clearly see it is a puppet.

As Underdog is digging to bury the bomb he sets it down to dig, but in the next scene he has it in his mouth while digging.

Dan Unger puts his coat on twice in the same scene.

The original sweater that Jack gives Underdog is knit with a stitched in "U".

When Polly calls Underdog, we can see that his sweater is no longer knit, and the "U" has an orange outline and is not stitched to the sweater.

When Jack is speaking to his dad near the end of the movie, Underdog is listening from behind a wall.

In the previous scene he was wearing his new collar, but when he is behind the wall the collar is missing.

The next time we see him, the collar is back.

Early in the movie, when the mailman is taunting Shoeshine, he is seen driving an older style jeep DJ5.

All post offices now use a Grumman LLV which was put into service in 1987.

The dog that they use for barking at the mailman is obviously not the one that is used in (most) of the rest of the movie.

If you look closely at the patterning around the face, you'll see that on the barking dog's left cheek, the brown mark doesn't touch the lip, whereas the "main" dog's brown mark does.

Also the barking dog has a softer outline.

Underdog catches on fire upon re-entering the earth's atmosphere, yet his sweater and cape go unscathed.