In physics instruction we often begin by presenting students with an abstract principle, and then illustrating the principle with one or more examples. We hope that students will use the examples to refine their understanding of the principle and be able to transfer the principle to new situations. However, research in cognitive science has shown that students' understanding of a new principle may become bound up with the example(s) used to illustrate it. We report on a study with physics students to see if this "specificity effect" was present in their reasoning. The data show that even students who understand and can implement a particular physics principle have a strong tendency to discard that principle when the transfer task appears superficially similar to their training example.