Can't believe I never saw this thread before -- what an excellent topic! I think my biggest mistake of all time was buying into the idea that I should be very careful about using fertilizer because I would just get leaves and no fruit (tomatoes, peppers, etc)... then I started being much more liberal with the fertilizer, on the recommendation of our conservation district guy, and have had pretty tremendous results.
My biggest mistake this year was to plant a lot of my onions inside my "hoop trellis" that I grew gourds on... they by far didn't receive enough sunlight and my onion harvest this year was quite pathetic! Oh well...
