I'm wondering if anyone taped the episode "When Aliens Attack" and then slowed down the tape during the battle scenes - because I suspect that the animators may have snuck in a variety of unusual ships.
Just about everyone caught the school bus, and at least one person spotted an older Starfleet ship.
But I could've sworn that after the real mothership uncloaked and began firing, that one of the ships destroyed was the Mystery Machine!
I wouldn't be surprised if they snuck in some other gags like that in those shots.
(A similar thing happened in an episode of Tiny Toon Adventures, in which Plucky Duck became Pluckzilla - a jet began firing missiles at Plucky, and if you slowed down the tape you could see other things being shot at him, including a kitchen sink, an anvil, and George Jetson.....)