While soldering a board together for eval purposes I wasted over an hour fixing it. +/-2.5V supply...one of the bypass caps was connected incorrectly (it was polarized), and I had a feedback resistor shorted over a trace rather than across the appropriate pads.

One of those days...

 

I recall the heads shaking at a Heathkit store about a guy who managed to assemble his color tv kit with the components all on the foil side of the boards.