I think my biggest comeback was last season, when I turned a 20-point 5x5 deficit into a lead of 10-15 points in the final month (against Dan S., no less
). It was a just-for-fun league, and I had saved up a huge chunk of my IP cap in the early going while relying heavily on hitters ... you can guess the rest. Spot starters can go a long way.
Anyway, it was just an experiment, but an interesting one.