There was a particular moment when I realized I was very much a junior developer.
Now, in school, I have done very well. I do well in my classes, I'm proud of the code I write, and when I've worked with others I think I've been a productive and active team member.
But working in the real world is different. I'm no longer the big fish in the small pond.
I was working on a bit of code, and I wasn't sure exactly how to do what I wanted, so I looked on Stack Overflow. Someone else had asked exactly the same question, and the answer had exactly the code I was looking for. I copied and pasted, and included a link to the answer in the comment.
On review, a senior developer pointed out that there were copyright issues with that. I asked legal, and the short answer was "no, you can't do that." So, I asked a different senior developer -- one who hadn't looked at any of the related code -- to reimplement it. I explained what I wanted and gave links to relevent parts of the API documentation (but not the SO answer).
When he finished and asked for a review, I was stunned. His work was better than mine in every way. He solved the problem more completely, more elegantly, and more thoroughly. He started with less information and less context and he wrote something amazing.
I have a long way to go.