If you toggle the gravity of an ArmorStand via the setGravity method it is possible for it to fall through the world under certain circumstances:

Way 1:

1. Set the NoGravity tag of an ArmorStand via /summon or /entitydata to 1b
2. Use a plugin to setGravity(true) on the ArmorStand
3. Watch it float through the world into the void

Way 2:

1. Set the NoGravity tag of an ArmorStand via setGravity(false)
2. Leave the chunk with the ArmorStand (let it unload, if it doesn't unload it works fine)
3. Return to the chunk and setGravity(true) on the ArmorStand
4. Watch it float through the world into the void

Please note the previous issues which seem to be similar but occurred always (SPIGOT-508 and SPIGOT-657) and that this behavior does not appear if you only use the vanilla commands.

(Bug also happens on git-Spigot-fdc1440-53fac9f)