Of course it is.I broke so many strings over the years.The funny thing is when it happens on o top pianist's concert.It shouldn't happen too often, but it happens sometimes.
If you brake some, don't worry, it's not a big deal.They cost from 5-30 euros and you can change them by yourself.
I am doing Grade 7 at the moment and my teacher always tells me I play to loud. My Grade 5 examiner also took away 5 marks in one of my pieces for the same reason.Then a few days ago a string broke on my piano but I was able to get it fixed the day after so it was no big deal!