Basically when something Fanon like a theory, character, idea, etc gets introduced to a fandom that people think that the Fannon character, theory, idea, etc is canon. Hence the name CANON FANON EFFECT. A "canon" theory, character, etc.
There are two variations to look at this effect. Oblivious or Disregard
Disregard: When people are aware that this character, theory, idea, etc isn't canon. But treat said character, show, theory, idea, etc as if it was canon.
Example: The Creepypasta Fandom often draws Slender man with the operator symbol in the background or most times anything really related to Slender man and sometimes even referrers to it as the operator. Even though the fandom knows that the operator and Slender man aren't the same entity.
Oblivious: When someone isn't aware that said idea, theory, character, etc isn't canon.
Example: The Creepypasta fandom thinks that Eyeless Jack has an origin in which he was used in a sacrifice to become a monster. Even though he never had an origin, to begin with.
Guy1: Hey man your drawing sans eye wrong. It supposed to be glowing with a flame.
Female1: Actually sans never had shown to have a glowing or flaming eye, just one that switches colors really fast.
Guy1: Wait, really? then why does the fandom draw it whenever he's mad or using gaster blasters?!
Female1: Must be the Canon Fanon Effect.
It is said by famous tiktoker @loamfy on every meme of a game called criminal case, if you’re gonna use it credit tiktoker @loamfy
Peter: *shows loamfy a criminal case meme*
Loamfy: canon I work for pretty simple
A thicc boy named Owen who canon has fantasies about. They have a very steamy relationship because both are quite busty.
Canon Robin’s Boyfriend loves his massive shlong.
Not necessarily canon, but canon in your own perspective. Your own alternate perspective of what’s canon.
In my head-canon, Mario in the original Donkey Kong games from the early ‘80s is actually the father of Mario and Luigi. That Pauline is probably a different Pauline. Maybe it’s modern Pauline’s mother. The second Mario in Donkey Kong Jr. is Mario and Luigi’s random uncle.
A canon pairing in fiction is a couple accepted as official by the author and a large part of the fanbase.
Eren and Mikasa from Attack On Titan are a canon pairing
Da most popular light-classical selection dat da local fast-food joint plays over their Muzak speakers.
I love soft chamber-music as much as anyone, but it can still get a trifle tiresome hearing dat same cloyingly-sweet-toned Tacobell Canon in D played at least once over da diner's PA-system on every occasion while I'm chowing down my favorite lunch.
The act of pooping liquid on the male partners belly whilst performing doggy-style.
My wife totally let loose with a gravy canon last night, after she got shit-faced on tequila!