This case study focuses on the feedback that is provided by tutors to learners in the course of synchronous online teaching. More specifically, we study how trainee tutors used the affordances of Visu, an experimental web videoconferencing system, to provide negative feedback. Visu features classical functionalities such as video and chat, and it also offers a unique marking tool that allows tutors to take time-coded notes during the online interactions for later pedagogical remediation. Our study shows that tutors mainly use verbal and chat feedback, with significant inter-individual variability, and that tutors who provide verbal feedback are more likely to use markers. Marking takes time because of the dual task that it entails for the tutor. Idiosyncratic strategies in the use of markers are evidenced. These results clearly show the value of markers for negative feedback, signal the need for their explicitness, and also call for an evolution of the Visu interface so that tutors can better negotiate the task of online tutoring and the pedagogical stance they have to take on in their interactions with the learners. (Contains 5 tables, 3 figures and 2 notes.)