What I expected to happen was...:
Experience orbs not rendering their texture's transparent pixels at all.
 
What actually happened was...:
Some experience orbs render their transparent pixels as black instead when looking in certain directions.
 
Steps to Reproduce:
1. Create a world
2. Throw lots of bottles o' Enchanting
3. Notice that when you look in certain orientations, some of the experience orbs appear with a black background