In S07E10 of Futurama, the Professor returns to his childhood home where he has "Transformers" bedsheets.
In Silicon Valley, the binary on the screen has a hidden message for viewers saying "find a hobby for god's sake."
In Community, when Beetlejuice's name is said for the 3rd time in the series, he appears in the background.
In S04E14 of Gotham, after Edward Nygma comes out of the ice and isn't smart anymore, he points to his eyes and ears in the wrong order.
In S05E12 of Bojack Horseman, Diane is eating at an Italian restaurant with a picture of the leaning tower of Pisa in the background slanted to make the tower seem straight.
In the South Park episode "The Red Badge of Gayness," both sides in the Civil War reenactment fly Confederate flags.
In Simpsons, the reason Mr. Burns says "Ahoy Ahoy" when answering the phones is because Alexander Graham Bell wanted people to answer that way. This is just a way to show how old he is.
In S02E02 of Smallville, Clarke Kent's mum is carrying hot trays with oven gloves and Clarke helps out using his bare hands, small detail showing his powers.
In S10E18 of Simpsons - "Simpsons Bible Stories", all of the boys are shirtless but Milhouse who wears the girls uniform.
In S05E16 of Friends, Ross says that he and Rachel did it 298 times. This means that Rachel gets pregnant with Emma on the 300th time.