Trial SS Serpent Sting Macro

Grumble has been playing around with macros and I think he is on to something. He wrote up this macro to cast Serpent Sting if it isn't on the target, Steady Shot, and Kill Shot at the end. I knew he was up to something because he was out DPSing my Steady, KS macro and he was gloating that he had Viper on most of the time and not paying attention to switching aspects for this run.

/castsequence reset=target Serpent Sting, null
/castsequence reset=3/combat Steady Shot, Arcane Shot, Steady Shot
/cast [target=pettarget,exists] Kill Command
/script UIErrorsFrame:UnregisterEvent("UI_ERROR_MESSAGE");
/cast Kill Shot

Line one will cast Serpent Sting is there isnt one already on the target.
Line 2 repeats SS, AS, SS and will start over if you pause for more than 3 seconds.
Line 3 casts Kill Command if it is up.
Line 4 is to prevent error messages from popping up.
Line 5 casts Kill Shot if the mob is under 20% life and it is up.

The only time I get any kind of error sounds is when I first start and am trying to cast Serpent Sting. I havent gotten any errors such as Spell Not Ready. We have seen a sharp increase in the damage we have been doing.