RE: What you believe is commonly overlooked when tuning or troubleshooting.
from what I gather if you have two bare shafts and the weights vary, so will the spine. Esecially with carbons. So if you weigh your arrows and there is a large weight difference with one of them and you rule out the tip or insert (you could do this by checking foc), chances are the spine will be off as well. Of course if your tip weight is off it will effect your spine as well, I don't think to the same degree though.
Those are very good points about varifying your head weights. Every good archer should own a grn scale to weigh thier arrows and components with. I have found most components to be within better than a half a grn of each other. Most of the weight variances I see are within the shafts themselves.
Paul