Everyone makes mistakes - including Feynman

Early stages of the theory of radiative corrections to weak interaction processes such as muon decay and pi - e decay are described based on my personal recollection. The discovery of an error in our initial paper on the muon decay played a crucial role in the realization of remarkable cancelation of mass singularities in integrated observable quantities. General formulation developed to deal with mass singularity turned out to be very handy for numerical evaluation of high-order radiative corrections to the lepton anomalous magnetic moment. This has led to the most stringent test of QED available at present. New developments of last two years are described briefly in 'Note added in proof'.