What I expected to happen was...:
To kill a mob and see/receive experience orbs.

What actually happened was...:
I attacked and killed several mobs and different types of mobs (creepers, zombies, pigs, etc) and although I'm pretty sure I gained the experience and heard the sounds of collecting them, many of the orbs would not display or show up or would intermittently display.

Steps to Reproduce:
1. Find a mob.
2. Slay it.
3. See if experience orb displays and shows correctly until collected.

I also tried changing all of the different graphic settings in Minecraft, to no avail that I could see.