I don't know what triggered this and I've not seen it before.
I, finally, logged my rogue in and I was near a hostile NPC which attacked me before everything had loaded.

Edit 1. It's something to do with the rogue class, I haven't spent any talent points yet and won't until I've tested some more.

Edit 2. It only happens in Assassination or Pirate (Outlaw) spec.

Edit 3. Disabling all other addons except Bugsack/Buggrabber it only errors in Pirate spec. I'm very confused now.

[string "=[C]"]: in function `min'
[string "@EventHorizon\EventHorizon-1.10.1-beta.lua"]:1899: in function `f'
[string "@EventHorizon\EventHorizon-1.10.1-beta.lua"]:806: in function <EventHorizon\EventHorizon.lua:797>

Locals:
(*temporary) = nil
(*temporary) = nil
(*temporary) = "number expected, got nil"

It looks like this was caused by the Pirate config still having a recharge instead of a cooldown.