Killing Sinding in his cell before he escapes (Ill Met by Moonlight)
You can speedrun the whole quest if you kill and skin Sinding when you find him in jail; you just have to put a rune spell on the wall and activate it after Sinding gives you the Cursed Ring of Hircine, otherwise the quest will fail. Using a sword or a bow can make Sinding's corpse fall down the stairs which makes it impossible to skin/loot. You should wait for him to finish walking up the stairs and exit conversation with TAB after selecting "I'll take the ring to Hircine", then activate the rune with fire.