So I'm somewhat anal about tuning and will think nothing of spending 5 minutes ascertaining that each string is indeed in tune to the last Hz. After a typical painstakingly drawn out tuning session, I put on some backing music and dig in. ****! It sounds weird instantly. I pluck each individual string, and find nothing remotely dissonant,. It takes a few seconds before I realize...the second string is tuned up to "A."