Continuity mistake: The hammer on Leela's blaster changes from smoothly curved to angular.
Continuity mistake: As Bender is stomping to stuff the whale down the sewer, when the rope gets to the pavement, the top where the cover is tied suddenly jumps from the level of Bender's head to below the spot where his legs and body meet.
Where the Buggalo Roam - S4-E6
Continuity mistake: When Kiff is being offered the peace pipe, in one angle there is smoke coming out of it, in another there is no smoke.
Continuity mistake: When Bender laments that he'll be drifting through space with just a little swag, he takes a gold wristwatch out of the bag and puts it on. The watch disappears a few seconds later when he pulls the little piano out of his chest storage and is never seen again.
Continuity mistake: When Bender hits the asteroids, one pierces his head. Seconds later that asteroid is gone. Where did it go?
Continuity mistake: Near the end, when Leela talks to the second female blurnsball player, her black nose-stripe disappears then reappears throughout the shots.
Where No Fan Has Gone Before - S4-E12
Continuity mistake: When the Planet Express Ship first lands on the planet, it crashes with noticeable damage. When they try to sneakily escape while the screen play is going on (before they get the parts from the Star Trek ship), the damage is repaired.
Answer: You are thinking about episode 20 of series 2 - "Anthology of Interest I". She technically never kills anyone, but in the episode she ask a "What if machine" (one of the Professor's inventions) what would happen if she was more impulsive. The machine then shows the more impulsive Leela killing off the Professor to get the money from his will and thereafter killing Hermes, Bender, Scruffy, Cubert, Nibler, Amy and Zoidberg to cover it up. Finally she does something really impulsive with Fry.
Andreas[DK]