As far as I can see, this bug is out of our hands, and yet another (new) Blizzard bug in the SecureGroupHeaders.lua file:
http://eu.battle.net/wow/en/forum/topic/2844894567
The division by zero errors are all related to UnitPowerMax() and/or UnitPower() sometimes returning 0 for non-combat/vanity pets on the PTR. Which is why it seems to be random, as it seems to only happen the instant a pet gets into your range. At least this one is something we can easily fix!
