I worked on a main service panel today that had two failures.

1) All of the receptacles on one wall in the living room were dead.
The failure: The black conductor mounted to CB 11 broke off where the cable
Went under the breaker screw.

2) The washing machine receptacle receptacle only showed 37 volts.
The failure: The Neutral conductor for CB #13 broke off at the screw on the
Neutral bus bar.

I thought it was odd and merited a question

The conductors broke I will bet anything due to a nick in the metal when stripped and then stressed by the tightening of the screws, and possibly heating and cooling effect.