Some vignetting in the corners can be "fixed" in post processing. A vignette with a negative intensity value (i.e. brighten instead of darken) can be applied to the image. It won't fix extreme cases but it may help a little bit.
I shot a roll of Fuji Velvia in my Spotmatic. The lens hood clipped the corners but I didn't notice it in the viewfinder. After scanning the film I applied the negative vignetting and rescued a little of the image corners. What is totally black is gone though. That's OK. It still made for decent shots I think.
IMGP0542 by
Never Off, on Flickr
The corners are still a little dark but much better than before. Applying any more negative vignetting made the edge-center look too bright.