With lower consumer prices and the boom in solar PV module sales there has been a rapid increase in the number of PV modules installed on roof tops. In some instances, noticeable changes in the visual appearance of these modules have been observed, such as the occurrence of 'snail trails' or microcracks or discolouration. These changes raise concerns that the modules may be defective or not performing as warranted. We have examined a number of these problematic modules and identified several common defects that have appeared on installations around Australia and the effect that these defects have on the output of the individual solar modules. Results from a series of studies of modules showing these defects have been systematised and presented in this paper. (C) 2013 Elsevier Ltd. All rights reserved.