Sometimes absolutely everything goes right. The holes are perfectly aligned, the tenons correctly sized, the back is beautifully symetrical, and yet somehow, one of the spindles is bowing out and jamming you in the back.
I guess that at some point with nothing left to lose, I started using this jig to correct errant spindles.
First, I heat up the offending spindle in the area that will take the bend. I am careful to heat it slowly and without getting close to scorching it. Once heated, I simply use a hand clamp and my parenthesis shaped board to pull the spindle into the correct position (actually past it to account for spring back). You can see that I cover the ends with foam to keep from marring any finished surfaces. I have been able to do some dramatic alterations with this method (sometimes more than one heating is necessary) and the results have proven to be permanent as far as I can tell. Good Luck.