I love this description by Neil Gaiman of his experience of imposter syndrome early in his career, quoted in Presence by Amy Cuddy:
I would have this recurring fantasy in which there would be a knock on the door, and I would go down, and there would be somebody wearing a suit not an expensive suit, just the kind of suit that showed they had a job and they would be holding a clipboard, and they’d have paper on the clipboard, and I’d open the door and they’d say, “Hello, excuse me, I’m afraid I am here on official business. Are you Neil Gaiman?” And I would say yes. “Well, it says here that you are a writer, and that you don’t have to get up in the morning at any particular time, that you just write each day as much as you want.” And I’d go, “That’s right.” “And that you enjoy writing. And it says here that all the books you want they are just sent to you and that you don’t have to buy them. And films: it says here that you just go to see films. If you want to see them you just call up the person who runs the films.” And I say, “Yes, that’s right.” “And that people like what you do and they give you money for just writing things down.” And I’d say yes. And he’d say, “Well, I’m afraid we are on to you. We’ve caught up with you. And I’m afraid you are now going to have to go out and get a proper job.” At which point in my fantasy my heart would always sink, and I’d go, “Okay,” and I’d go and buy a cheap suit and I’d start applying to real jobs. Because once they’ve caught up with you, you can’t argue with this: they’ve caught up with you. So that was the thing in my head.