Praise be to Allah.
If the matter was as you described, there is no sin on you because you did not do that deliberately, but you are liable for the damage you did, even if the cause of that was the parking attendant who was organizing the parking of the cars. That is because you are the one who actually did the offence, whilst he was the cause of it, and the basic principle is that when there is both someone who was indirectly the cause of an offence and someone who actually did it, then the one who actually did it is liable.
It says in al-Mughni (8/344): When there is someone who actually did an action as well as someone who was the indirect cause of it, liability is borne by the one who actually did it, not the one who was the cause of it, such as when there is a digger and a pusher. End quote. What is meant is: such as when there is one who dug a well and one who pushed someone into it; in that case liability is borne by the one who pushed the other person, because he is the one who actually did the action.
And Allah knows best.