[Haleth]

Alright, with that extra check everything seems to work now. I've added a simple check for the bank too and it all seems to be going nice and smooth, but I'll let you know if it still messes up.

This is what it now looks like:

function CheckSlots()
	if GetContainerNumSlots(4) == 0 then
		if GetContainerNumSlots(3) == 0 then
			if GetContainerNumSlots(2) == 0 then
				if GetContainerNumSlots(1) == 0 then
					NUMBAGS = 1
				else
					NUMBAGS = 2
				end
			else
				NUMBAGS = 3
			end
		else
			NUMBAGS = 4
		end
	else
		NUMBAGS = 5
	end
end

function CheckBankSlots()
	NUMBANKBAGS = GetNumBankSlots() + 1
end

local f = CreateFrame("Frame")
f:SetScript("OnEvent", CheckSlots)
f:RegisterEvent("PLAYER_LOGIN")
f:RegisterEvent("PLAYER_ENTERING_WORLD")
f:RegisterEvent("BAG_UPDATE")
f:RegisterEvent("VARIABLES_LOADED")

local g = CreateFrame("Frame")
g:SetScript("OnEvent", CheckBankSlots)
g:RegisterEvent("PLAYER_LOGIN")
g:RegisterEvent("PLAYER_ENTERING_WORLD")
g:RegisterEvent("BAG_UPDATE")
g:RegisterEvent("VARIABLES_LOADED")

CheckSlots()

Going to do some cleaning up when I see that everything works fine after a while of using it. I don't really need the two frames either.