When the yarn snapped I didn't cry. I didn't even curse. I looked at the hole and the running stitches, stopped what I was doing, walked to my bag, got some yarn and a needle and went back over to the shawl. I grabbed the three runaway stitches and faked some little looping around thing. Then I left it alone. This morning when it was dry I took a better look at it. I can see one end where the yarn broke but I can't see the other. I didn't want to start tugging and poking at it so I just did some more semi-duplicate stitching around on the back where it broke and wove in my ends. I think it's all right. I sure won't stretch this scarf out again though. It will have to live it's life a little short and slightly unblocked.
In retrospect I was surprisingly calm. Like a person who has just cut off his own arm with a chainsaw but still has the presence of mind to dress the wound and call for help. Just like that.