Like for example a ML/FL in high school brags about having the perfect GF/BF and then the lie becomes truth, or the FL/ML has an "imaginary" BF/GF who suddenly becomes a real person or similar.

Or if it has to be with an occupation or whatever else non-romantic, I guess I'm okay with that too =)

Basically what I'm looking for is a great lie that becomes truth.

Maybe Dear Diary? FL writes a journal about her love interest and he then comes to life in her world.

"Best Mistake":

To get a boy from her past to leave her alone, Kim Yeon Do uploads a photo of a random guy claiming him to be her boyfriend. However, she does not realize the photo she chose was of bad boy student Ji Hyun Ho.