Abstract The term ‘epistemic trespassing’ has recently been coined to denote a person’s judgments regarding a domain where they are not epistemic experts. In this paper, I focus on expert trespassing testimony – that is, testimony by an expert in a domain of expertise other than his own. More specifically, I focus on intra-scientific trespassing testimony between scientific collaborators. By developing a number of distinctions, I argue that while intra-scientific trespassing testimony may seriously hamper scientific collaboration, it does not invariably do so and may even be beneficial to it.