Easter egg: On Disc 4 of the Complete Fourth Season, select the Extras menu for Krusty Gets Kancelled. On this menu, press '35', 'Enter' and a pencil will appear in the bottom right corner. Press Enter to view some secret sketches from the episode. (Note: on some remotes, to enter '35' the +10 button may need to be pressed first.).
Easter egg: On Disc 3 of the Complete Fourth Season, select the Extras for Marge vs. the Monorail. On the episode screen, press '1' then 'Enter' on the remote, and then '50' and 'Enter' (some remotes may require the +10 button to be pressed for this). A microphone will appear in the bottom right corner. Press Enter to view the episode with a secret second commentary.
Easter egg: On Disc 3 of the Complete Fourth Season, select the Extras for Homer's Triple Bypass. On this menu, press '2', 'Enter', '40', 'Enter' to make a pencil appear in the bottom right corner. Press 'Enter' to view some secret sketches of Homer. (Note: for some remotes, to get the '40' the +10 button may need to be pressed.).
Answer: I'm not sure if this is the one you're thinking of, but an episode of "Ray Bradbury Theatre," called "A Sound of Thunder," dealt with a similar matter: a group of hunters travel back in time to hunt dinosaurs, only to find things have changed when they get back because someone stepped on a butterfly.
Xofer