Despite widespread recognition of the need to improve assessment in higher education, assessment tasks in individual courses are too often dominated by conventional methods. While changing assessment depends on many factors, improvements to assessment ultimately depend on the decisions and actions of individual educators. This paper considers research within the "heuristics and biases" tradition in the field of decision-making and judgement which has identified unconscious factors with the potential to limit capacity for such change. The paper focuses on issues that may compromise the process of improving assessment by supporting a reluctance to change existing tasks, by limiting the time allocated to develop alternative assessment tasks, by underestimating the degree of change needed or by an unwarranted overconfidence in assessment design decisions. The paper proposes countering these unconscious limitations to change by requiring justification for changing, or not changing, assessment tasks, and by informal and formal peer review of assessment task design. Finally, an agenda for research on heuristics and biases in assessment design is suggested in order to establish their presence and help counter their influence.