Or when they want years of experience with a specific framework, library, or other system.
I remember looking at Java positions last year (before I got my current job), and a lot of them would sound nice until I got to the item about "Must have X years of experience with Spring".
Yeah, I have a whole bunch of core Java under my belt, and I made the requirements for everything else, but because I don't have experience with that one library, I'm shut out.