Special-case analysis--setting parameters to special values and checking that the answer is consistent with previously known results or physical intuitions--is a common strategy for reflecting on the correctness and meaning of answers to physics problems. We interviewed eleven calculus-based introductory physics students to learn about their use of such reflection strategies. Six of the students were enrolled in a reformed course where students were prompted to reflect on their homework solutions. Checking special cases was specifically suggested in the homework instructions as a reflection strategy. Five students were in a different course that did not prompt reflection on homework. During the interviews, none of the students performed a special-case analysis or were familiar with the strategy. We suggest that students need explicit instruction on how to do a special-case analysis if they are expected to perform it while reflecting on answers to physics problems.