For want of a nail the shoe was lost.
For want of a shoe the horse was lost.
For want of a horse the rider was lost.
For want of a rider the battle was lost.
For want of a battle the kingdom was lost.
And all for the want of a horseshoe nail.
This rhyme has been repeated for centuries as a lesson in the possible consequences of even the slightest bit of carelessness. And the writer on AH.com whose handle is TheScottishMongol has applied to the world of A Song of Ice and Fire with a fan-fic of the same name.
Basically Catelyn Stark's horse loses a shoe on her way back to Winterfell from her visit to King's Landing and the resulting delay means she never encounters Tyrion Lannister at the inn. She never arrests him on suspicion of his involvement with the near-death of her son Bran and he makes his way to King's Landing, where he meets the new Hand of the King Ned Stark and discovers that Littlefinger has slyly accused him of involvement in the attack on Bran.
Now a man who's a great warrior but has little political savvy and a small man who casts a very large shadow are thrown together and some very interesting things result. These include a wonderful Crowning Moment of Awesome that I'll sum up in a few words--"Trial of Seven." Although the story misses some steps, it's a lot of fun and certainly worth a read.
If you liked this story, the second one is posted here.