Callback

Sometimes a joke or some other element from a previously produced segment is referred to. We call this a Callback.

Baby Terminators - The final scene in this sketch calls back to Kid Venison.
Behind the Music: Electric Mayhem - During Dr. Teeth's interview, a copy of Ode to the Nut Shot sheet music can be seen behind him.
Where Is He? - Calvin was about to shoot Waldo calls back to Our Newest Member, Calvin.
Berry Rude - This segment uses the same music as You Got Robo-Served.
Bloopers! Two - Katie is reading a story about an incident from the previous Bloopers! segment.
Cultivated Relationships - Makes reference to the previous Battlestar sketch, Frakking Galactica.
Dance Dance Counter-Revolution - Castro's line "I looove doing that" after shooting the boy calls back to the end of Walt Disney Attacks!.
Djinni Mermaid - M. Night Shyamalan appears, exclaiming his catch phrase "What a Twist!"
Final Fantasy Burger Chain - Two characters reenact the exchange from I'm So Hungry.
Getting to Paris is Hard - There is a poster of Hannah Montana and Wildman.
GI Joe's Fight for the Weather - Opens with a clip from Come and Get Some! and a presumed clip from The Diary of Anne Frank.
Grandma Fu - One of the old men makes the same "Gheeee!" sound first heard in Ironic Heart Attack.
Jedi Master George W. - Ends with a callback to the very first Robot Chicken sketch, W. Loves Tacos.
Heroes with Time Machines - Images from The Diary of Anne Frank can be seen in the film on the stage.
Horny Robot Callback - Yes, the infamous robot is still humping that washing machine.
Horny Robot Redux - He's back again, and expanding his tastes!
Just the Good Parts - Ends with a callback to Cultivated Relationships.
The Lion, the Witch and the Locker - The same unicorn that appears in Unicorn Mayonnaise.
Our Newest Member, Calvin - has with a callback to Where Is He?.
Little Drummer Goku - Features characters from A Very Dragon Ball Z Christmas.
Paris and Nicole's Prison Break
The Pen Lying On Your Desk - Among bathroom stall graffiti is a hung TV-headed person. This character was also used as the icon in Top 100 Final Episodes Ever.
Redshirt Revenge - The final line, "That's good ham!" is a callback to Care Bear Genocide's "That's good rainbow!".
Robot Chicken End Credits - Includes a callback to Jar Jar Returns.
Robot Chicken Sells Out - Contains a number of callbacks to other sketches, including Horny Robot, Where's Michael? and Eagle Eye Smith.
Sabrina Exhumed
Season Four
The Secret Life of Jack-O'Lanterns - The "ass-crackers" crack is from Huggytime Bears.
Dora the Disaster - Features the Grim Reaper from Super Heaven.
Forrest Gump's Son Has New Legs - Features Tiny Tim from Scrooge's Gift for Tiny Tim.
Quiet Place Urkel - Features Chloe from Fourth Wall Rehab.