The one and only time I broke a string was when I was in a bit of a hurry and not paying close enough attention to my tuning. I developed a habit of simply trusting my tuner pedal and thinking why bother with unmuting? I recall trying to bring the G sting up to pitch when all of sudden the G string snapped! I was shocked but immediately realized what I did wrong. I was just lookin at the tuner pedal on mute and not paying attention the sound of the string. I no longer do that.
I lost a nice set of strings that day but looking back on it, the lesson I learned taught me to not waste a set of new expensive strings. I now listen to the pitch as I tune and if anything I might actually improve my vocal pitch perception.
So snapped a string but in the end I learned a valuable lesson the hard way.
"When you learn it the hard way, the lesson is so much more reinforced.