Development of mushrooms is driven by genetic and epigenetic factors in a continuous interaction with the environment. It is assumed that each successive stage of morphogenesis depends on specific sets of signals arising at the appropriate time and place during the growth process. Morphogenetic dynamism proceeds in a time dimension through a cascade of signal-effect associations. Developmental errors may occur when such signals originate in the wrong place and/or at the wrong time. As a result various abnormalities such as ectopic tissues can develop and morphogenesis can be severely disturbed. Both endogenous genetic disturbances and exogenous factors can cause developmental errors. Lamellar dysplasia, which is a pore-like proliferation of the gills, forms an example; it may be induced experimentally. Both lamellar dysplasia and rosecomb disease of Agaricus bisporus result from endogenous genetic instability, whereas the developmental errors observed in wet bubble disease, which is caused by the infection of Mycogone perniciosa, originate from an exogenous factor. Morphogenesis normally leads to a symmetrical form of primordia of A. bisporus. Asymmetry is very frequently associated with an underlying pathological situation. Defining exact criteria of and sharp borderlines between normal and abnormal development seems infeasible. Fungi may readily tolerate morphogenetic imprecision. In this report, various macro- and microscopic features of normal and pathological development are illustrated; the dilemma of normal versus abnormal fruit body development has been discussed.