Verbalisation (or talk-aloud protocols) has been used as a technique for knowledge elicitation in research for some time. It allows the researcher some degree of access to the reasoning behind participants’ decisions. The verbalisation technique is said to directly represent cognitive task processes while not affecting performance. Using a combined methodology of concurrent and retrospective verbalisation to study the effects of verbalisation on pilots’ task performance in simulator conditions, the findings of this case study suggest that, on the contrary, instructing operators to verbalise their thought processes while performing complex tasks may interfere with task-related cognition under some conditions, thereby compromising safety.