Michaelangelo says that they sent Slash into space in episode 74, but it was actually in episode 83.
When Raphael says "I wonder what Slash wants with the broadcasting antenna," he's using Leonardo's voice.
In the "Slash the Evil Turtle From Dimension X", the space shuttle Slash escaped in was last seen getting sucked in a portal to Dimension X. In this episode, Slash returns without a portal activation.
(Vernon and Burne are transformed back into humans)
Vernon: I'm human again. I'm normal.
Burne: Human? Yes. Normal? No-no-no-no-no.
Leonardo: (over turtle-com) Donatello, where are you?
Donatello: Under a pile of girders somewhere in the midtown area.
Leonardo: Do you need help?
Donatello: Does Michelangelo like pizza?
Episode included on the Teenage Mutant Ninja Turtles Season 6 DVD set.