The mysterious "Noodle Incident" is the Trope Namer. Infrequently, a character would reference a "noodle incident" that Calvin had been a part of, to which Calvin would immediately go into desperate denial or panicked defensiveness about. However, nothing about what happened during the aforementioned incident was ever explained throughout the strip's entire ten-year run.
The earliest reference to it (September 14, 1990) was when Calvin's mom goes to a parent-teacher conference, and Calvin is dreading her return. When she gets home she catches him packing, which prompts him to immediately protest that everything was a lie, at one point bringing up "the noodles", which his mother had not heard of yet.
Calvin: She told you about the noodles, right? It wasn't me! Nobody saw me! I was framed! I wouldn't do anything like that! I'm innocent, I tell you! Mom: What noodles? Calvin: Oh... Uh... Ha ha! Did I say noodles? You must have heard wrong. I didn't say noodles.
According to a later Christmas strip (Dec. 12, 1990), even Santa Claus doesn't know exactly what happened, or whether or not Calvin is innocent, despite having magical surveillance systems. He probably knows the broad outline, of course, since it's not that people don't know the details in-universe.
Calvin: I can't tell stories! Hobbes: What about your explanation of the noodle incident? Calvin:That wasn't a story! That was the unvarnished truth! Hobbes: Oh, don't be so modest. You deserved a Pulitzer.
One early strip mentioned the equally ambiguous "Salamander Incident". There, he claims temporary insanity.
In one storyline that involved Rosalyn, Calvin's mom says she called seven other people and none of them agreed to babysit. She then tells Calvin, "You remember Amy? She just laughed when I called her." It's anyone's guess as to how Amy's night with Calvin went.
At the end of the arc where C&H accidentally push the family car into a ditch, Calvin comments about how surprising it was that his parents didn't yell at him for this. Hobbes starts to say, "But try keeping live worms in your dad's..." Calvin snaps, "Let's not talk about that, OK?!"
Much like the "Noodle Incident", Bill Watterson has deliberately withheld any details about Hamster Huey and the Gooey Kablooie. All we know about it, other than the title, is that it includes a "Happy Hamster Hop", which Calvin’s Dad despises just as much as the story itself.
There's a substitute teacher who appears in only one strip and is referenced in onlythree. She's introduced in the middle one, and in the last one, Calvin is walking with Hobbes and mentions that she left at noon.
In this Sunday strip, Calvin tries to become a human kite by tying the kite string around his waist and having Hobbes fly him, but he's too heavy. They decide he needs to get lighter, and in the last panel, Mom is on the phone with the neighbor, who's complaining about a naked kid running around in her front yard, tied to a stuffed animal. Dad just says, "You handle it. I got the little nudist out of her bird bath, remember?"
A pair of early strips that precede the first mention of the Noodle Incident is seen by many fans as the beginning and end of the actual incident: The first strip has Calvin putting on a space helmet saying "today I go for the gusto!" The next day's strip has Hobbes asking Calvin about his day at school, to which Calvin angrily says he doesn't want to talk about it. Hobbes asks if it was related to the sirens he heard, and Calvin shouts "I SAID I DON'T WANT TO TALK ABOUT IT!"
There was an early Sunday strip note The one that starts with "Hey dad, remember our car?" where Calvin tries to get advice from Hobbes about what he should do if, hypothetically, he did something bad to the family car. Hobbes quickly realizes it's not hypothetical at all and the last panel shows them making plans to flee to Mexico. We never do find out what happened.
The arc wherein Calvin breaks his dad's binoculars. Calvin claims that he dropped them as he was tossing them to himself as he was running down the sidewalk...which Hobbes and the readers later seesomehow resulted in the binoculars being reduced to a fine powder. It's up to the reader's imagination just what exactly happened between Calvin initially dropping the binoculars and the start of the arc.
Calvin: Don't sneeze.
One strip has Calvin and his mom coming home from the movies, asked by Calvin's dad how it was. Calvin somehow seems to have barely registered the movie and his mom furiously says they're getting a video player. That's all the hints there are to what happened.
mentioned the equally ambiguous "Salamander Incident". There, he claims temporary insanity.