Triangular structural defects, occasionally generated during long term operation of 4H-SiC pin diodes, are known to negatively affect the forward characteristics of the diode. We have used synchrotron white beam X-ray topography, scanning electron microscopy, in situ cathodo luminescence and transmission electron microscopy for characterizing the structure and formation mechanisms of such defects. It is shown from high-resolution images that the defect results from glide slip on the (0001) basal plane. The defect consists of a stacking fault bound by two partial dislocations with Burgers vectors 1/3<11 (1) over bar0> and 1/3<01 (1) over bar0>. The fault is a means for stress relaxation in the epilayer, near the contact layer using an existing dislocation as a nucleation source.