dr. strangelove
no scene has left more of an indelible impression on me than the scene in dr. strangelove where slim pickens rides a nuke.

Death of the Simpsons
I can pinpoint the day that The Simpsons became irrelevant, and I think everyone can agree with this: May 28, 1998. Phil Hartman died that day and The Simpsons hasn't been the same since.