The goal of the study presented in this paper is to discuss how knowledge of formal logic changes the way students produce and validate proofs in the context of undergraduate mathematics. With that in mind, we asked 8 students with varied levels of knowledge and different academic background in formal logic to produce and validate proofs through a task based interview and we analyzed their work. In particular, a nonroutine task was proposed and showed interesting work from the students. Our empirical results suggest that a course in logic changes the way students do mathematical work in many ways. For example, it creates alertness to logical characteristics and a need to rely on the context.