Research has shown that students in inquiry-based physics labs often expect their experiment to verify a known theory or model, contrary to the goals of the lab. It is important, therefore, to identify ways for instructors to shift students' expectations or epistemic frames to those in line with scientific inquiry. In this paper, we analyze video recordings of one inquiry-based lab session in which the instructor intentionally encourages students to falsify, or disprove, the claim under investigation. We find that students operationalize the instructor's prompt by taking up one of two distinct epistemic frames: open outcome and verification. Students in the open outcome frame initially expect to falsify their claim, but form other conclusions in the face of alternative evidence. Students in the verification frame, however, view falsification as verifying that a claim is false and do not consider other possible outcomes even when they find conflicting data. These results suggest that students may interpret instructor prompts for frame shifts in very different ways. We argue that to shift students to epistemic frames in line with scientific inquiry (e.g., the open outcome frame), instructor prompts should explicitly address uncertainty in outcomes (regarding an experimental result as unknown) and epistemic agency (perceiving oneself as a producer of knowledge).