https://github.com/kemayo/wow-silverdragon/blame/58d8259d9a0e2b812326d838e7f84b9d76d285ac/loot.lua#L113

ContinuableContainer:AddContinuables() - plural, is not a valid method in Cataclysm Classic (and from looking at source not in upcoming 4.4.1 either)

I had to change the relevant snippet to

if #continuableLoot == 0 then return callback(loot) end
local continuableContainer = ContinuableContainer:Create()
for _,continuable in ipairs(continuableLoot) do
  continuableContainer:AddContinuable(continuable)
end
continuableContainer:ContinueOnLoad(function() callback(loot) end)

to make it stop erroring any time I kill a rare.

Well, that's a delightfully silly inconsistency. Thanks for pointing it out, I will fix that.