Just saw this post. Great responses from everyone above.
I always carry extra strings both in my case and in every performing jacket I have. Need to remember to empty my pockets when I bring my jackets or tux to the dry cleaners.
If you are a leader and your string breaks it is traditional to pass the violin back and someone gives them yours. That is not always so sure to work and is never well planned or rehearsed but it did happen to me once. I was just handed another violin without reaching out.
I only broke an E string another time in an orchestra performance and improvised way up on the A string until the piece was over then I quickly reached in my pocket and exchanged the broken string for the good one during the applause.
On stage with the electric violin it has happened many times with my Zeta because the bridge has metal pickups under all for strings. Metal on metal makes for quicker wear and tear especially on the E string.
When this happens I can't grab the other violinists violin because all parts are equally important so I usually crack a joke about what just happened and the rest of the group kind of plays a loop, kind of like an introduction, and I impress the audience by replacing the string super fast and usually get a nice applaud afterwards.....