Official episode discussion: Rise of the Turtles, Part 1 and 2

[Lisardo]

Originally Posted by The Big Bad

I'm a bit surprised that this remains a point of confusion twenty-five years after the fact. Sure, it doesn't all occur on-screen, which is sort out of character for the writers, but I think the sequence of events is clear enough:

1) Yoshi has been with the rats off-screen when he sees the turtles covered in mutagen.

2) Yoshi steps on the mutagen in order to get to the turtles, activating his mutation to rat.

3) Yoshi begins cleaning the turtles. Since his own mutation to rat has already begun, touching them does nothing.

But...he touched the turtles last. So why did he not become a turtle (that's how the mutagen works, the last thing you literallly touched)? That's the part that (if you nitpick) doesn't make sense. Truthfully, there is no explanation. It's just a an eff-up on the writers behalf.