Accidentally triggered the game-breaking scenario in Legend of Zelda: Skyward Sword? Nintendo will fix up your save file with a dollop of magic.
In this case wizardry comes in the form of the handy repair staff working for Nintendo - you're now able to send your save data directly to the team to get it all patched up. The support page also reveals that those very same technical folk are hard at work to release a program that will do the hard work for you, to prevent the laborious task of having to send over the file to Nintendo.
The bug happens if you decide to complete a set of tasks later in the game in the wrong order, triggering the game-breaking disease by innocently talking to a chap twice.
How to prevent this situation from occurring:To complete the Song of the Hero quest, the player must visit three different locations to collect three dragon songs. To prevent the situation where the game can no longer be completed from happening:
Complete the quests in Faron Woods or Eldin Volcano first. Do not go to the Lanayru Desert first.
If the Thunder Dragon's Song from the Lanayru Desert has already been collected first, do not talk to a Goron named Golo until the other songs have been collected. (Golo is found at the entrance to Lanayru Mine.)
What prevents the game from being completed?
In the quest for the Song of the Hero, if the Thunder Dragon Song in the Lanayru Desert is collected first and Golo the Goron is spoken to before the other songs are collected, the cut scenes that start the sequences to get the songs in the other areas won't occur. This prevents being able to make any further progress in the game.