In the Beetle Bailey strip seen here, we can only imagine what happened at the party that Mrs. Halftrack is referring to.
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 Great Tea Trolley Diaster of '69 in Bristow. What is known is that involved massive loss of life, and resulted in several employees recieving awards for bravery.
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.)
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!"
In the popular comic strip Pearls Before Swine, Pig is enemies with a sea anemone due to an unknown event that occurred years in the past. It's a Feghoot for "Annie-May, my anemone enemy".