THEA is a technique developed to help designers in interactive systems (originally in the aviation domain, but hopefully applicable in other contexts) to anticipate interaction failures or “human errors that may be problematic once their designs become operational. The techniques is intended for use early in the development lifecycle, as design concepts and requirements concerned with safety and usability, as well functionality are emerging. This report uses examples from two flight deck based case studies to illustrate how to use the THEA technique for carrying out a human error analysis during early design.