There is growing interest in implementing reform-based lab courses in undergraduate physics that are student driven rather than instructor driven. In these courses, students develop and carry out experiments while simultaneously reasoning about their hypotheses, data collection procedures, collected evidence, and the relevant physics content. However, there is a limited understanding of how students reason in these types of labs. Using the theoretical framework of sensemaking, we examine qualitative observational data of undergraduates engaging in open-ended experimentation in reform-based introductory physics for life sciences labs. We examine the moment-by-moment details of students' sensemaking by focusing on a series of inconsistencies, specifically focusing on what the inconsistencies are about and what moves students enact during sensemaking to resolve them. We explore sensemaking about conceptual and procedural inconsistencies through a narrative case study analysis of a single group whose sensemaking is largely representative of the observed data corpus. We find that students engaged in sensemaking to resolve conceptual inconsistencies by juxtaposing hypotheses and evidence and critiquing and constructing scientific explanations, sometimes evoking elements of mechanistic reasoning. Comparably, we find that students engaged in sensemaking to resolve procedural inconsistencies by proposing and testing a series of causes towards modifying experimental procedures or apparatus. Overall, we find that student sensemaking about both types of inconsistencies is generally productive, given students' resolutions and experimental progress. Capturing detailed sensemaking about inconsistencies highlights the richness of students' reasoning processes in this understudied learning environment that is becoming more prevalent.