I've never broken a spoke (front or rear) in 50 years of cycling on/off road. But last summer I did witness another rider's rear spoke break when the guy I was following up a wet & slippery 20% slope suddenly tried to downshift (no idea why he wasn't already in his lowest gear by that point) and then stomped hard on the pedals. His back wheel momentarily slipped and then when it regained a bit of traction - crack!