Scalar diffraction theory is usually presented as a closed subject in current textbooks. Actually, the theory is extremely unsettled and open to original investigation, and the authors feel that student understanding and motivation could be significantly enhanced if the problematic nature of the theory were stressed. With this in mind, the Kirchhoff, Huygens and Rayleigh theories are reviewed, emphasizing the fundamental inconsistencies and contradictions which are often ignored. Instructive and provocative experiments are then described which confirm the unwarranted success of the theoretical solutions under extreme conditions. The investigations are based on the diffraction of microwaves by a circular aperture, and are on a level suitable for undergraduate work.