Enter: Mutagen Man is a season 5 episode of the 1987-1996 series.
A delivery boy accidentally falls into Krang's new unstable Mutagen and becomes able to transform into anyone he chooses. Shredder tricks him into helping him launch the Mutagen into space and mutate the world, but this "Mutagen Man" has other ideas.
Seymour is animated mirror-flipped in parts of Act Two.
When the Transport Module burrows underground when Shredder and the mutants head for the space station, the mud splatter is the same color as the floor surface the Module was standing on.
Shredder is talking to Kraang on the Technodrome's display monitor at the beginning of the episode. His right hand and part of his clothing can be seen underneath the Technodrome's control panel after delivering his first line.
Right before Seymour turns himself into Michelangelo for the first time, one of the "n's" on the "Channel 6" sign is missing.
When the construction workers gang up on Leonardo and Raphael, Raphael is seen with Leonardo's colors.
When the turtles find April trapped on the sky tram, they arrive with Raphael on the very far right next to Leonardo. But in the next shot, Leonardo is suddenly on the far right next to Raphael.
Michelangelo: Who the heck are you, Muta-dude?
Seymour: I am Seymour Gutz, mild-mannered delivery man although my friends say I'm lots of laughs at a party!
Raphael: Nice going Donatello, we almost get thrown in jail because you wanted to look at some oversized Fourth of July rocket!
Leonardo: There's been a mistake, we thought you needed help!
Worker: Yeah, well you look like alien invaders to us!
Raphael: Alien invaders?! Don't you guys watch television?
Workers: Sure! And the first thing you aliens do is learn our language! (fires off nail gun at Raphael and Leonardo)
Seymour: Excuse me! I'm Seymour Gutz from "Fly By Nights Messengers". Is this the "Touch of Romance" flower shop?
Shredder: Does this look like a flower shop?'...
Shredder: How the devil did you wind up all the way up there?
Seymour: Well, I don't have a good sense of direction.