This depends on many things, such as what the assignment is, how important it is, what year of study you're in, which university you're at, whether you've done anything like this before, how important the text you meant to cite is to the assignment overall and whether or not it seems like you did it on purpose.
Also, there's the possibility that they might not notice at all if you're lucky. At my university, I would perhaps expect to lose a couple of marks for an incomplete bibliography and there be no more said about it. I really wouldn't worry too much if I were you.