You have to pay attention to spot this one... On the episode Bad Girls, the Mayor became invincible and he proves it by letting a vampire attack him, after this, Mr. Trick (the vampire who works for the Mayor) slay the vampire on the back but if watch it closely you'll see that he stab him on his right side instead of his left side as it should be.
Although a common mis-conception, the heart is actually in the centre of the body (horizontally) so every "slay" on Buffy would probably miss the heart.
It must be said though that the heart is only slightly to the left of center, the weapon was reasonably thick, and so it would still be possible for the sword to hit the heart, or if not the heart the lung
Look at the episode Homecoming. The female vampire Candy was staked in the stomach with the wooden handle of a spatula and still dusted. The heart is no where near where she was staked yet she still dusted
![[Slipups.com Logo]](/images/slipupslogo_small.gif)
