Whenever i get into a minecart and attempt to:

-Fire an arrow(this is a bit spotty, I need to be looking somewhat downwards for it to impact the cart and break it)
-Throw any potion
-Throw a bottle o' enchanting between the y-rotation values of appx. 114 and 154
-Throw a snowball(same results as arrows)

Then the projectile will hit the minecart. If the projectile is a bottle o' enchanting or potion, it will hit the northwest corner of the minecart as if it had struck a block. For bottles of enchanting, I found that I needed to look northwest between the horizontal(y-rotation?) values of approximately 114 and 154 for it to strike the minecart. Otherwise it would behave as normal. No matter which way I looked when throwing a potion in the minecart, it would still impact the northwest corner. It's how I killed myself(and discovered the bug) on my survival world when I attempted to throw a potion of harming at a zombie as I was riding past in my minecart. It somehow hit my minecart and I died. Ouch.