I am so fearful of situations like this that I don't do high level crafting with rare mats - pathetic but true! I definitely think there should be a possibility to return to your guildmaster & say "I screwed up, can you desynth this into the rare mats?". After all, they're supposedly the best in the land so why can't they unpick a leather waistcoat to retrieve the correctly cut pieces & the thread that sewed it together?