Is there a name for things that appear in all or most episodes of shows that you can look for just for fun?
For example, the bunnies in The Secret Show or dandilions other than the ship in Cosmos?

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"If you think the LHC will create black holes, you might as well believe Hobbits are at the bottom of your garden."- Dr. Mike Inglis
Rovers forever! - ToSeek
"Carl Sagan sent a message to ET,
Neil Armstrong walked in the Sea of Tranquility
Steve Squyers built Spirit and Opportunity
Dan Haylen upchucked in zero gravity." -Brent Simon, The Space Camp Song

Easter Eggs.

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Spock Jenkins of the Vulcan Jenkins'.

Running gags.

Fred

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"For shame, gentlemen, pack your evidence a little better against another time."
-- John Dryden, "The Vindication of The Duke of Guise" 1684