In the final Thieve's Guild quest, you're tasked with a seemingly dangerous escape in which the game recommends you equip the Boots of Springheel Jak (enchanted boots provided by the quest that add 50 to your acrobatics). Doing so will protect you from the fall due to a scripted event but the boots are destroyed in the process. However, should your actual acrobatics skill be high enough and/or you have sufficient health, the fall is easily survived without them and you can keep them permanently if you don't equip them before you jump.