Fanfiction term for when a Mary Sue or Gary Stu or self-insert not only enters a story and becomes Ms/Mr. Wonderful, but the entire story centers around said Mary Sue or Gary Stu bashing or insulting the canon characters to accomplish its own superiority.
Derived from the terms "party crashing" and "gate crashing," in their meanings of unwanted access to an event by someone who should not be present.
The following passage from a fanfiction that shall be nameless illustrates canon crashing.
Wolfwood looked at her. “Absolutely not! Vash and I are going after a man who just blew up the Second Moon!”
“Which is why,” a deep baritone voice thundered as a huge hand reached down and easily lifted the heavy cross up, “this should be left to us professionals!”
Wolfwood turned about to see Harrisburg holding his Punisher up with one hand. It popped open and deployed its machine gun.
“My, you’re a big one,” he said with a wide grin on his face and sweat rolling off his brow.
Harrisburg grunted. “Nice box gun you have here mister… Let’s go.”