"Screw history!"

Farnsworth begs everyone to not do anything to severely damage the timeline throughout most of the episode. The nearby Roswell Air Base has the machinery they need to get home, but the Professor is adamant that they mustn't steal it, as they have to preserve history the way it was. However, after Fry accidentally kills his grandfather and sleeps with his grandmother, thus becoming his own grandfather, the Professor decides that the damage has been done and storms the air base.

Example of:
Screw This, I'm Outta Here