Blizzard has confirmed that some players have received the Warlord's Deathwheel mount today after a bugfix was implemented. According to Blizzard, all players affected were players who originally unlocked the mount but, for one reason or another, did not receive it back in 2014.
We looked into this and yes,
The reason was something like a bug or crash that was invoked the instant that the mount was initially supposed to be awarded, and then a fix for that as a result of a recent update.
And there may be other achievements that a very small number of players have been missing, until now.