You asked for no onions but got onions? It's a conspiracy! What if you were highly allergic?! They are trying to kill you!
You should stomp in, demand to speak to a manager and then proceed to scream and call everyone idiots while throwing in some derogatory racial comments.
I'm sorry, what's that? You'll come off looking like a psycho if you do that?
Taco Bell employees make minimum wage. I'm sure they would have happily made you a new 89 cent burrito if you would have kindly explained what happened.