It depends if I have corroborating information.
Normally, it means that they are lacking some very fundamental piece to their learning. The last class I taught that this happened in had a particularly poor Data Structures teacher and so didn't understand Hashes/Dicts/Associative Lists and how to use them.
Yeah, that assignment didn't go well for about 30% of the class. I had to back up and teach that piece.
However, my first assignment ALWAYS has 40+% failure rate. And my assignment is: "Here are the directions to: set up Eclipse, set up Mercurial, type in this program, compile it, run it, and check it in so I can run the automated tester on it."
I literally give people a document with every single step documented. I'm not joking. We're talking a single print statement that prints "Hello, World!".
40%+ always fail. It's almost invariant. Nothing I do can budge that number. I want to bang my head off the desk at the office hours just before and after that assignment.