I recently purchased a Sigma 30mm f/1.4 lens for my Canon 450D and noticed soon that at least wide open, any bokeh circles in the lower half of an image (in landscape orientation) seem to have their topmost part cut off. The upper half seems to be fine (not demonstrated in the image, sorry.)
At first I thought that maybe there's an aperture blade that does not retract completely, but based on visual inspection that does not seem to be the case. Might it be the mirror box itself that's the culprit?