Detective Conan - After giving a lengthy rant on subjects a six year old definitely shouldn't know about, Conan covers up (or at least, tries to) by saying "At least, that's what it said on TV!" (or something similar).
Tonto says this when he is called out by the Lone Ranger in Dynamyte's comic book series.
Preacher: Cassidy, the Vampire — "This gravy tastes like semen.." Seeming to understand the implication of his choice of words hastily adds "Or so I'd imagine". This becomes a Harsher in Hindsight moment when Jesse learns more about his friend's history than he'd care to know.
Mr. Deeds: features Deeds telling his butler, Emilio, that he dreamt about a girl he liked, after an entire night of drinking booze. Emilio anwers, "Usually, when you're black out drunk, you don't dream" which elicits a suspicious stare from Deeds. Emilio then explians "... so I read".
"They're the very devil to forge, I know that," said Moist. "Or so I've been told," he added quickly.
Live Action TV
Subverted in an episode of Happy Endings, Max, Brad, and Penny complain about 'thumb-face' Larry. Brad doesn't like how he forcibly looks at his shirt tags, Penny doesn't like his repetitive stories, and Max has this to say...
Max: Yeah, plus he's bad in bed. *Shocked looks from the others* I've heard. From people. Who are me.
Ashes to Ashes: "It's like something out of a German porn film... apparently."
Father Ted: Uses this in one episode, wherein the titular character and his fellow priest, Father Dougal, accidentally find themselves in "Ireland's biggest lingerie section, I understand. Yeah I read that... somewhere." They then proceed to meet a number of other Priests who have "accidentally" found themselves in the store, and the scene turns into a war film parody as Ted leads the band to escape out the fire exit without anyone noticing half a dozen priests surrounded by ladies' intimate items.
Torchwood: uses this in the episode "Sleeper," when the Monster of the Week comments on Jack's bedside manner, Gwen responds "You should see his manners in bed, they're atrocious. Apparently, so I've heard..."
Ianto: Oh, they are. I remember this one time -
Jack: (coughs loudly)
In the NCIS episode "Singled Out", McGee describes a speed dating service as "designed to introduce successful men to a wide assortment of eligible women. Each night brings the promise of romance and the chance of finding your soul mate." His team mates look at him questioningly and he hurriedly adds "... or so I've heard."
Bones: Not referring to a work of fiction specifically, but does reference this.
Booth: Yeah, well, a woman finds out a man is cheating on her, she can get really angry. (Others stare at him) That's what I heard, okay?
Alexis also does this when speaking about a specific scene from a horror movie she's not supposed to have seen, fairly unsubtly.
Alexis: If I'd seen it, which I haven't.
Even Castle himself does this once, regarding a phone sex line:
The phone sex line dispatcher: It's not just about the sex. That's the part of this business most people don't understand. Guys call, girls call; they're looking for release. Sometimes, that release is sexual, but sometimes...
Castle: It's therapy. Beat Or... so I've been told.
On an episode of Law & Order, Briscoe tells Curtis about a dockside location where detectives in the old days would go to waterboard difficult suspects. He immediately follows up with "or so I've heard."
In Boy Gone Astray an ex-boyfriend of ADA Connie Rubrirosa tells her and her boss information on various drug cartels. Quickly adding "So I hear" after each piece of information.
Cops L.A.C: note The Australian police drama, not related to COPS the American Reality TV show., the detectives find a flyer for Romnett in a murder victim's house. A junior officer standing nearby tells them it's an internet dating site, "I've been told."
One character on Little Britain is a retired actor who always steals his sister's Meals on Wheels dinners. He is very bad at hiding at, as he tends to praise the meals to the delivery boy's face, adding "Kitty said," when he realises he's almost given himself away.
Pharmacy employee: He looked a little young for a penis pump.
Detective Fin Tutuola: One of those vacuum tube things they sell on the back of magazines to make you bigger... (seeing Stabler's smirk) or so I've heard.
Another episode had Fin telling the others about the lifestyle of black men who have gay sex on the D.L. while living straight lives with wives and children. He then responds to their weird looks with "Don't look at me,; I just know stuff."
Landlady: This is Ron Vibbentrop. Mr Johnson: Oh, not Von Ribbentrop, eh? "Ron Vibbentrop":(shouting) Nein! Nein! (calms down) Oh. Ha ha. Different other chap. I in Somerset am being born. Von Ribbentrop is born Gotterdammerstrasse 46, Dusseldorf Vest 8... so they say!
During the opening of one episode of The Colbert Report, Colbert gives "a shout-out to all of my Bronies", followed by an insincere "And I do want to reiterate that I do not know what that means."
In an early episode of Seinfeld, Jerry and Elaine are planning to see a film together, but when Elaine is busy, Jerry watches the film without her knowing and later tries to get out of seeing it again.
Jerry: It's so bad, there's no story, it's really boring . . . I've heard.
In an early episode of Angel, Doyle tries to talk Angel into wearing a magical ring that makes him invulnerable to sunlight, and comes out with:
I know a couple of strip clubs that have a fabulous luncheon buffet ... I mean, it's ... I've heard.
A variation appears in Golden Girls, when Dorothy asks her prospective daughter in law's mother if it's true about black men in bed. Blanche shouts "Oh yes, definitely!" When everyone stares at her, she amends it to "Oh yes, definitely that is something I would like to know too."
The Devil in Old Harry's Game does this whenever he is describing one of his own deeds while disguised as a mortal.
In an episode of I'm Sorry I Haven't A Clue, the panellists are playing a game based on Who Wants To Be A Millionaire...except all about hats.
Humph: Who is the odd one out of Henry Ford, Fred Astaire, Winston Churchill and Abraham Lincoln? ...I'm sorry, Tim, the answer is Henry Ford. His hat size was six-and-seven-eights, and the others were all seven-and-one-eighths. And I have to tell you that these are all true, I don't believe this—six-and-seven-eights?
Tim: That's tiny! That's a schoolboy's size! ...I'm told.
Optional dialogue in Mass Effect 2 between Mordin and a krogan scientist. The real kicker is that Mordin actually did make the krogans more infertile by upgrading the genophage when it looked like the krogan were recovering.
Mordin: Salarians created genophage - I've heard. Still trust them with medicines and agriculture?
Black Mage of 8-Bit Theater has an unfortunate habit of doing this, as he tends to forget that the mass-murderous evil he so gleefully perpetrates is rather frowned upon by the general populace. For instance, when questioned by the city guard, he carefully details his flippant murder of a cobbler and only realizes after the fact that he'd all-but-confessed before attempting a recovery.
Black Mage: [...]Of course, that plan assumes that my knife can cut through bone, which as I discovered last night, it can't. ...Er, hypothetically speaking, as I don't have any knives.
207. The Chicken and Rice MRE is *not* a personal lubricant. (Skippy wanted this noted for the record that this is not something he has ever attempted or considered! It was something we heard at dinner on 22 September 2001 and it was just so obscene it had to go here.)
209. An airsickness bag is to be used for airsickness *only*. (Also not a Skippy-ism...this was the same dinner.)
South Park episode "The Return of the Fellowship of the Ring to the Two Towers" plays this for laughs; all the men in town only have to hear the title "Backdoor Sluts 9" and they all know exactly which porn it is.
Mr. Stotch: Backdoor Sluts 9 makes Crotch Capers 3 look like Naughty Nurses 2!
Mr. Brovloski: It is the most depraved, vile porno ever made...
Mrs. Brovloski: And how do you know?
Mr. Brovloski: I, uh, read about it in People.
The Fairly Oddparents: When Wanda takes over her father's business, she put a ficus in the meeting room. One of the men comments on a gardening tip for it, then quickly invokes this trope when the others stare at him surprised.
"A ficus? That's gonna need more light... not that, I would know."
Not only has he said this about at least 2 other things, he later says "I hear the ficus is doing quite well" *everyone looks at him* "...or so I heard", the name of the trope.
In "The Tale of the Dash of the Dark" on Peter Rabbit, Benjamin says this when Peter's sister Cottontail is having trouble sleeping and he says that nightlights are good for helping little kids to get to sleep.
Actual quote from a review by Germany's leading culinary critic Wolfram Siebeck:
"The soup tasted the way I imagine packet soups would taste."
Sometimes, this can happen when trying to teach Martial Arts to others.
"Yeah, if you cut the quadriceps right here, the other guy will drop like a rock."
"... Not that I would know personally, though."
Let's face it. You've probably used this at some point during your life.