One of my kills "Hazzor the Viper" (Act 2, Sewers of Caldeum) was unchecked after having gotten the kill earlier today. Verified the tick after the kill.
Was just now browsing through other people's achievements (for said achievement), went back to look at mine and it had unchecked the kill. Took a bloody long time to find, please fix.
Edit: Appears it was a display bug. Tried logging off and on and it's there, ticked again. *Phew*