Three Men and a Comic Book - S2-E21
Trivia: Bart is surprised to learn that the actor who played Fallout Boy hadn't died in Vietnam. This is a reference to when both AP and UPI erroneously reported that Jerry Mathers of 'Leave it to Beaver' had died in Vietnam.
The Simpson's 138th Show Spectacular - S7-E10
Trivia: The credits read "Written by Penny Wise" and "Directed by Pound Foolish."
Homer's Barbershop Quartet - S5-E1
Trivia: This is the only episode in which Homer is voiced by someone other than Dan Castellaneta; his singing voice is done by a member of the Dapper Dans.
Trivia: The hair salon marge goes to is called Hairy Shearers, a reference to Harry Shearer.
Cape Feare - S5-E2
Trivia: Bob's prison number is A113. This number is often used as by alumni of California Institute of the Arts (most famously by Brad Bird, in his Pixar films and episodes of the Simpsons he was involved in, such as this one) - it's the number of one of the classrooms.
The Principal and the Pauper - S9-E2
Trivia: Martin Sheen was chosen to voice the "real" Seymour Skinner because the producers had admired his role in Apocalypse Now, and thought his voice would be appropriate for a Vietnam veteran.
Trivia: Paul McCartney agreed to guest star on the condition that Lisa's conversion to vegetarianism remain permanent. This is one of the few points of continuity that has been strictly adhered to.
Tree House of Horror IV: The Simpson's Halloween Special IV - S5-E5
Trivia: When Marge asks Bart to explain how scary the episode is, she mentions the "War of the Worlds" broadcast on NPR. Narrated by Orson Welles, that radio serial was so chilling, a number of listeners actually believed Earth was under attack.
Tree House of Horror XII - S13-E1
Trivia: In "House of Whacks", the murderous house is disengaged when its British charm unit is removed; Pierce Brosnan, the personality whom the family had chosen for the house, is actually Irish.
Trivia: One of the sliest jokes in the series' entire run is the name of talk-show host Opal's boyfriend, Straightman. Steadman Graham, the longtime beau of Oprah Winfrey (on whom Opal is based), is rumored to be gay.
Trivia: When Homer sees Moe as the little bird in the cuckoo clock, it keeps repeating "cuckold" instead of its customary cuckoo call. Homer then asks, "Whats a cuckold?" The term is generally applied to a husband whose wife is sleeping with other men. Perhaps fitting, as Homer suspects her of having an affair.
Trivia: This episode contains pictures of Peter Griffin from Family Guy & Stan Smith from American Dad.
When You Dish Upon a Star - S10-E5
Trivia: Another thing "The Simpsons" predicted: Towards the end of this episode, a sign outside of a film studio shows the 20th Century Fox logo with "A Division of Walt Disney Co" written under it. Nineteen years after this episode aired, Disney purchased many assets from 20th/21st Century Fox, including "The Simpsons."
Trivia: Smithers made his first appearance in this episode but he was mistakenly animated with the wrong colour. He was made into an African American with blue hair. This was corrected in the episode "There's No Disgrace Like Home", where he appeared in his normal yellow colour.
Trivia: Dan Castellaneta and Hank Azaria both voice about ten regular characters, with Azaria contributing another ten recurring, and Harry Shearer voices twelve regulars.
Sideshow Bob's Last Gleaming - S7-E9
Trivia: When we are first introduced to Sideshow Bob, he is building a ship out of sticks. Look at his name tag. It says A-113 on it. A-113 is the classroom number used by character animation students at CalArts, frequently used as an inside joke by graduates, right the way up to Pixar movies.
Trivia: In a recent episode (I saw it when it premiered on 5/17/09) the Odgenvillians move into Springfeild, then are forced out. When Lenny and Carl go on the chair attached to balloons, you can see the house for Pixar's latest film, Up, in the background.
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