there was a fairly young and thriving internet company that i thought would be the bomb to go work at. They pulled me in for an interview, but as soon as I was done, they announced that the interview was officially over, but asked me would i mind taking a consumer survey for their marketing efforts.
I took the survey which was all about what kind of products i use with an online connection, and it was obvious they wanted to know which lines of business to expand into online. Didn't get the job and I suspect that i only was interviewed as a way to get me in for their marketing survey. But that is okay because that employer is notoriously bad to work for, so I hear.
Another job i applied for, I knew I bombed because the main person on the interview team started mocking a mannerism I had just done, and I could tell, he was spoofing me.
But to the OP, don't worry about it. I know for certain that many jobs that are advertised are designated to be "inside" jobs - meaning, they have to go through an interviews as a matter of protocol, but they already have someone in mind, often an internal candidate, who they intend to hire. So, every interview is good for building experience at it, and don't think twice about a rejection.
There's also a whole strategy you can use about a follow-up letter which can be a nice touch, and even a de-brief if you don't get the job so you may gain insight on what to do differently next time.