The Jeff Foxworthy Show has an in-universe example. Jeff and his college professor father-in-law Elliot are dressed as a gladiator and the devil respectively trying to milk a nervous cow, Don't Asklong story. Elliot comes up with the idea of relaxing the cow by sing the Indianan University song. When Jeff explains the incident to his wife she says "I remember hearing my father singing that through my bedroom wall as a kid." She and Jeff look over at her parents and realize why her father was singing it.
Arrested Development - in one episode, Barry Zuckercorn makes an offhand comment like "Why do I keep getting all these bruises?" Easily dismissable, right? Not so much when you recall Barry's habit of picking up prostitutes (of both genders, apparently) and have just watched Philadelphia.
The fourth season of Nash Bridges had a Running Gag where a "phantom disco" (complete with strobe lights) in the unit's headquarters (a former nightclub) would come on occasionally without warning or explanation. The problem is finally fixed in season five which is a good thing because season six sees the introduction of Antoine, an epileptic. Had the problem not been fixed, the poor guy would have had a seizure every time the strobes came on.
An episode of Canadian children's show Ready Or Not has preteen heroine Amanda exploring her Jewish roots. At one point, she's horrified to see that the synogogue has been vandalized, a swastika has been spraypainted on the side and she hears somebody yelling anti-Semetic slurs. The kids she's with don't seem to have much of a reaction at all and her classmate calmly tells another kid to tell the Rabbi. When you think about it you realize that this is probably because it happens so often that it's an accepted part of Jewish life.
Fringe establishes in the season 3 episode "Amber 31422" that people trapped in amber are conscious the whole time. Then in season 5, Walter, Astrid, Peter, and Olivia are all revived after being encased in amber for 20 years. Walter has brain damage and doesn't remember it, and what about the others? It helps explain the difficulties between the reunited Peter and Olivia, who were having a rough patch before being separated. They have been thinking about the occupation, the resistance plan, and the safety of their missing daughter for 20 years and couldn't do a single thing about it. It's never mentioned in the series, but it's there in the background making season 5 even grimmer than before.
Plus, Michelle Hathaway is a freaking lunatic. How is the world is this nutcase a mother?
The Big Bang Theory Sheldon, his Eidetic memory is played for laughs, but the horror comes when you realize he remembers EVERYTHING with excruciating detail, including every last hurt, every last insult, every last time he was picked on as a child, which happened a great deal, and he can remember it all as if it just happened.
The Orville: In "Command Performance", Ed's joke to Bortus about eating one of his eggs. That's basically the equivalent of going up to a pregnant woman and telling her you want to eat herabortion.
Seinfeld: In The Pony Remark when Jerry gets defensive about inadvertently insulting Manya, he starts asking why someone would emigrate to America, a ponyless country, from a "pony-packed country". Manya was in her seventies, Jewish, and from Poland. It's a fair bet that she either left to flee the Nazis, or left after the Second World War to escape the bad memories.
Another from Seinfeld: Wilhelm seems just as zany as everyone else, until you get to The Bottle Deposit. He seems to have serious mental deficits as he is unable to remember things that he had done moments earlier; his wife notes that he had "forgotten to take his medication again". George's project led to institutionalization, it can only be assumed that Wilhelm completed it and had forgotten about it. In The Fatigues, Wilhelm gleefully announces George as his protege during George's Risk Management speech about Ovaltine...all signs point to Wilhelm suffering from Alzheimer's or dementia, making his zany antics seem all the more horrifying.
Speaking of George. . .in an episode where he sleeps with a married woman, the woman's husband declares his intent to "sew his ass to his face, break all his joints and reattach them backwards", etc. One wonders what kind of treatment his poor wife was in for when she returned home from their tryst.
The "Tale of the Pinball Wizard" episode. When Ross, the protagonist, finds himself trapped in a twisted version of his local mall, the first strange event that occurs is that thousands of quarters fall from the ceiling. When it's later revealed that he's trapped inside a pinball machine, this takes on new meaning: every one of those quarters is a credit in the machine, and thus a round of the game he has to play before escaping, if he ever can escape.
Peter is shown in "The Tale Of The Captured Souls" to spy on Danielle and her family through his mirrors as part of his plan to steal their youth. The cameras include views of the tub in the bathroom and the bedroom. Does this mean Peter has watched Danielle and her parents undress when changing, to take a bath, or going to bed? Especially alarming when one considers that he's implied to have disturbing Foe Yay towards Danielle.
Also, he has signs outside which have crossings on them to indicate how many lives he's taken to stay young. One sign depicts a woman, then a man, a kid, and a dog.
If Daniele hadn't stopped him she would've been an orphan and Peter would've continued to take souls to stay young if another family came to stay at his home, and would use Danielle herself if she refused to join him.
The ending of The Tale Of The Dark Music: Andy sics the monster on the bully that's been tormenting him all episode. However, he accidentally gets him eaten in the process. In return for this... the monster gives him a new bike (the bully destroyed his old one) and promises to give him anything he wants as long as he keeps feeding him. The final kicker, though? The last shot of the episode implies that he's more than willing to give the monster what it wants with a subdued Slasher Smile and that his sister is next to go.
Of course the nightmare is subdued when its revealed that one of the narrators mentioned that he didn't completely go through with feeding his sister to the monster.
Yet it still adds a dose of Fridge Horror when said narrator mentioned that he used the monster as a form of intimidation towards his sister. Which still screams He Who Fights Monsters territory.
Even if he didn't feed his sister to "It", I'd hate to be anyone who got on that kid's bad side. In fact, I'd hate to be anywhere near that kid whenever he wanted something that he somehow couldn't get on his own.
The various Monster of the Week beasts in Space Sheriff Gavan, Space Sheriff Sharivan, and Space Sheriff Shaider, while portrayed as usually goofy in each episode, have nightmarish implications if they were never defeated. The vast majority of which had at least one melee weapon they were at least semi-competent at wielding and their standard powers include shapeshifting, invisibility, teleportation, size alteration, telekinetic properties, and some bodily weapon; said powers increase in their home nightmarish dimension. These powers make most of them essentially minor deities each capable of ruling over humanity with little to no means of retaliating against them (aside from the titular heroes of course).
In the I Didn't Do It episode "Dance Fever," Sherri, an Ax-Crazy girl dangerously obsessed with being the only person with a perfect attendance record, purposely infects Lindy with the flu, and tries to make her trip down the stairs, poison her, and even attack her at home to get her to lose her perfect attendance record. She also injures Logan's date in order to be his replacement date and get into Logan and Lindy's house to terrorize her. Before locking Lindy in the janitor's closet, she admits that she did also cause the last person who lost their perfect attendance record, Greg Johnson, to become sick. Just how many people has Sherri infected, injured, or terrorized just to try and become the only one with perfect attendance?
In an episode of Family Matters, it's revealed that Urkel's locker combination corresponds to Laura's bust, waist, and hip measurements. How did he figure that out? There are many ways in which he could have done so, but everyone of them reveals him to be a obsessed pervert.