I think impostor syndrome comes in when we learn enough to realize that there's so much we simply don't know. It's sort of like how teenagers are so sure that they have all the answers, if the grown-ups would listen to them. Revisit those people at 30, and they'll be singing quite a different tune.
Personally, I've dealt with it when I start trying to seriously use a new language, or when I'm given an assignment that is difficult. Early on in my career (maybe 3 years in, so I wasn't a n00b), my supervisor came to me on a Thursday afternoon and said that he'd just come from a meeting, and he volunteered me to fix a bug in this program. The defect had been open for 6 months, and the 20+-year veteran wasn't making headway. He volunteered me - to have it done by Monday (end of day). This program was a language I knew (COBOL!), but it was written in a completely different style than I was used to, and came from a subsystem on which I had literally never worked before.
I didn't have time to think about all that, though - I had a deadline, so I had to use the tools that I had and see where they got me. Armed with a green-bar printout of the code, and the misbehaving report, I set to work. I read over the code Thursday evening, continued tracing it and learning about what it did on Friday, and found where the report was building this particular line. I determined that the number was correct up until they put it in the report. (Say what?) I didn't see what was wrong with it, but I knew where it was wrong, so on Monday, I brought my findings to one of our team leaders. He pointed out the problem (moving PIC 9 to PIC 9 COMP to PIC X, in case anyone is curious), and I made my deadline. (And learned about PIC 9 COMP in the process!)
Interestingly, I did not have the knowledge I needed to solve the problem when I started, but by using the knowledge I did have, I was able to pinpoint where the problem was, which led me to the knowledge I needed to acquire.
The mindset "I may not know, but I'm going to figure it out!" can be the difference between someone who suffers from impostor syndrome sometimes, or who is actually an impostor.
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