Utilitarians can distinguish between the goodness/badness of the act and the blameworthiness/praiseworthiness/responsibility/character/etc. of the actor.

So a utilitarian can coherently say: yes, it turns out that the person who pulled the lever (call him 'Bob') made the wrong choice. If we could go back and time and let him do it again, we'd say "Don't pull the lever Bob!"

But presumably Bob pulled the lever with the best of intentions, he couldn't have reasonably predicted the lever breaking, and there's no reason to think his decision reflects poorly on his character, judgment, future safety to others, etc. So there's no good reason to send him to jail, or shame him, or tell him he's a bad person.