I was visiting a guy who ran a shop with sales and instrument repair section. He was a very goos string player himself, having spent years in Symphony Orchestras. He was restringing a viola.

As he tightened the string, it would snap just above the nut. After the second one, I asked him whether the string holder (can't think of the name) was too far away from the end. He looked, cursed, removed the strings and shortened the cord so it was closer to the end, thus having a longer length of the metal string available.

Of course, it seems that some good quality string for the viola are hardened, except for the area where the string is wound around the tuning peg.