Thanks to my Friday critique group, I was able to save a painting that was teetering on the verge of extinction (I totally destroy paintings that don't please me!). Despite a few areas that worked, the painting as a whole was not good enough to keep. I've recently decided to become much more rigorous about the paintings I accept as finished.
'More active sky', legitimizing the sharp shadows across the road - both comments were key to the final version of the painting (see here).
Like anything else worth doing, more eyes (or ears or minds) turned onto a project can pick up difficulties, suggest solutions, see something that someone else - especially the person who created it - has missed. When I used to type for a living, proof-reading was the most difficult part of the job. I would always see what I intended to type, not necessarily what had been imprinted on the paper (see - the old days of the IBM Selectric II - workhorse of a typewriter).