I suppose it's not really a fallacy per se, but there's this delusion (?) especially among newer practitioners that I see get termed as "Main Character syndrome." They branch into magic, and then suddenly every crow is a personal sign from Odin knighting them as a chosen worshipper, every idle dream a message from some higher guardian, every bit of bad luck a powerful death curse sent by a hidden enemy, etc. They can work a bit of sigil magic and suddenly they think they're the centre of the world. It's less the Dunning-Kreuger effect and more people assuming that every event in the world around them is filled with important, personal meaning and they lack the discernment to see it's not. A duck is never just a duck, it's actually a djinn in disguise leading them to treasure or some other.
So, there are a lot of interesting corollaries to this trope of "main character syndrome".
I struggle with a few because I can have flashes of narcissistic thought that are painfully powerful at times.
I would add there are some people who do just the opposite: they accept that they are a side character of the universe but then forget somehow that they are ALSO the main character of their own life, and that this implies that if they wish to have supporting characters, the those people -- including they themselves -- must be good supporting roles.
They have to seek not to be the villain, not just of their own story, but of everyone else's too.
Because while we all love to hate the villain, we also love seeing the villain fail to continue to be villainous to people. Well, everyone except the villains I suppose.
Sometimes that means redemptions, sometimes that means "whatever games they must be made to play to learn how to play well with others" and for some, that's a very long and hard road, and will not happen for a
very long time.
Failing to understand that you are A main character in A story and potentially the villain in others, that's just as dangerous as thinking you are THE main character.