Seems to only happen while in a party

Message: Interface\AddOns\oUF_Phanx\Functions.lua:211: Usage: UnitHealth("unit")
Time: 11/04/16 18:16:33
Count: 7
Stack: [C]: ?
Interface\AddOns\oUF_Phanx\Functions.lua:211: in function <Interface\AddOns\oUF_Phanx\Functions.lua:206>
(tail call): ?
Interface\AddOns\oUF\events.lua:48: in function <Interface\AddOns\oUF\events.lua:46>
(tail call): ?

Locals: (*temporary) = nil

I just got the same error. It was during the Holy Priest artifact quest scenario, and it may have been triggered by one of the NPCs in the scenario leaving my party.

This should be fixed in oUF; if the release version from WoWI hasn't been updated, you can get the current development version on GitHub:

https://github.com/oUF-wow/oUF