Strings often break during quite moderate playing - hammers are the ones that will only go if you really whack them.
I broke a string during a piano-accompanied performance of Verdi's Requiem, quite near the end. It wasn't the fact that it broke that was the problem, is was the fact that it caused one note (a C somewhere in the treble, I think) to rattle very badly. It was a rather important occasion for emotional reasons and there was a very large audience, and I had to be ever so careful not to wreck everything by playing the rattling note during the quiet closing pages of the work.