In the strip seen here◊, we can only imagine what happened at the party that Mrs. Halftrack is referring to.
In another strip, Sarge is at his desk, and he's angry:
Sarge: Where's Beetle?? He can't just come in here two hours late with some cockamaimie excuse and expect me to listen! (Beetle enters wearing a sombrero, striped prison shirt, a life preserver, and lipstick on his face.) Sarge: I'm listening...
In one strip from Big Nate, Nate, Teddy and Francis are at a carnival and come across a ride called the "Screaming Meanie". This results in the following exchange:
Teddy: Ooh! The "Screaming Meanie"! Nate: Mister! Is this ride good? "Screaming Meanie" attendant: Good? Sure it is!... But I'm not at liberty to describe it in detail! Y'see, there's a lawsuit pending on account of some kid who tried this ride one time and was so traumatized... oops, I've said too much! Nate, Teddy and Francis: Three, please!
In another strip, Nate tries to convince Mrs. Godfrey to let him form a study group with Teddy and Francis, claiming it worked fine for midterms. To quote Godfrey, "You got a C, Teddy got a C-plus, and Francis got PTSD!"
A Bloom County strip had the middle two panels interrupted for an "editors note," leaving only the first and last panels free. In the first panel, Portnoy is asking Opus if he's heard the latest news: the last panel had Opus yelling "Turnips! Turnips and Antifreeze!" while Portnoy was yelling back "Not with Donny Osmond he won't!"
The British newspaper strip Bristow, set in an office, has the following examples:
It often includes references to "the great tea-trolley disaster of '67", since moved up to 2002 to cover the current occupants. What is known is that involved massive loss of life, and resulted in several employees receiving awards for bravery.
In addition, there is the "Great Luncheon Voucher Swindle of '68", possibly involving a younger son of Sir Chester Perry. This is spoken of in hushed tones and never referred to when the Boss is around.
Used in a Dennis the Menace strip in The Beano where a certain not fully explained and not shown incident involving microwaves, mobile phones and the fire brigade being the reason for Dennis not being left at home on his own.
The Far Side, as a one-panel strip, occasionally had installments where the joke was getting a glimpse into some ongoing situation with a background we're not made privy to. For example, one has a scientist who's apparently just escaped a sinking ship wash up on a "Far Side" Island to be confronted by a duck, who tells him: "So, Professor Jenkins, my old nemesis! We meet again... but this time, the advantage is mine!"
Gary Larson: What kind of sordid, bizarre past a scientist and a duck have in common is for anyone to surmise...
A homage to Calvin's Noodle Incident appears in one strip, where Jason remarks that Peter took away his Calvin and Hobbes collections after "that noodle incident".
In another strip, after Roger's Grilling Pyrotechnics manages to destroy a probe on the moon, Jason hands him the phone and tells him he owes NASA money. Roger's response? An incredulous "Again?!"
In an early Frazz strip, Caulfield asks if Frazz has any piercings. Frazz says yes, and that he ought to know where it is. Caulfield angrily responds with "How many times do I have to apologize for that stapler incident?!"
Garfield's owner Jon has tons of these, and he's certain to bring one up every time he opens one of his yearbooks, or family albums, or simply talks about his past. (The scary thing is, as goofy as Jon is, he seems to have an extended family full of relatives who are goofier.)
A girl named Ruby was once in prison. The only clue the readers get to her crime is Jon's decision to "hide the potato peeler" once he learns what she did.
In Get Fuzzy, the purported reason that the veterinarian doesn't make calls to the Wilco house is "the Hockey Stick" incident, for which Bucky is responsible.
A few other "incidents", like the Bag War, are expanded into full story arcs as extras for the back issue collections.
Nodwick has a lot of fun with these, with frequent jokes about offscreen adventures, which usually ended badly for Nodwick. Some others are more specific, and generally involve Yeager:
Exactly how it went between Snoopy and the Head Beagle when Snoopy was charged with "not pursuing his monthly quota of rabbits" was never shown. All Snoopy said was "fortunately, the Head Beagle was very understanding".
Speaking of which, the whole "Thompson is in Trouble" Story Arc was one Noodle Incident after another. It started with Snoopy getting a coded message from the Head Beagle saying that "Thompson is in trouble", with Snoopy recognizing the name instantly and remembering a past incident with "that stupid Thompson" that "almost got us all killed". (It's only confirmed later that Thompson is another beagle, but one can only guess what happened the last time, although Snoopy complains through the whole story that Thompson would "never listen to advice" and other such faults.) Snoopy leaves quickly to find this Thompson, does some investigating, questions a waitress in a restaurant "full of shady types" (who remembers Thompson), gets lost in the rain, and finds Thompson, but he's too late. The readers find out in the next strip where Snoopy writes his report to the Head Beagle that he had witnessed Thompson try to deal with ten-thousand rabbits by himself, and was presumably killed. (Again, it is not revealed why.) In the last strip of the arc Charlie Brown asks Snoopy if he thinks he'll ever know what happened, and all Snoopy can say is that "Those rabbits gave him an offer he couldn't refuse!"
◊, we can only imagine what happened at the party that Mrs. Halftrack is referring to.
