I've known this for a while and find it funny that Blizzard never actually finished registering these commands.
ChatFrame.lua:2320
Code:
-- easier method to turn on/off errors for macros
SlashCmdList["UI_ERRORS_OFF"] = function(msg)
UIErrorsFrame:UnregisterEvent("UI_ERROR_MESSAGE");
SetCVar("Sound_EnableSFX", "0");
end
SlashCmdList["UI_ERRORS_ON"] = function(msg)
UIErrorsFrame:RegisterEvent("UI_ERROR_MESSAGE");
SetCVar("Sound_EnableSFX", "1");
end