Before Dumbledore and Harry set off to go to the cave to find the Horcrux, Harry witnesses Dumbledore telling Snape "If something goes wrong here tonight, I give you permission to do it." However this creates a major plot hole for Harry. He doesn't know Dumbledore told Snape to kill him until Harry views Snapes memories in the 7th book. If Harry already witnesses this then after Snape kills Dumbledore Harry wouldn't accuse Snape at all.