Lost a child at dismissal

Subbed a Kindergarten class and at the end of the day I had a para who was there to help with dismissal and somehow one of the kids walked the opposite direction that he normally goes, then proceeded to get on a bus, he is not a bus rider. He was lost for 30 mins as he did not meet his brother at the opposite end of the building as he does every single day. Everyone was searching the building, the school grounds and they called parents and then finally found him on a bus. I feel so awful about it because I was to follow the dismissal plans but also the kids are usually so good about their routines, and i didn't double check who was going where. I'm so afraid I am going to get in trouble now. I feel awful about it. My anxiety about subbing was already high, and now this has just made it worse.

Edit: Update: Thanks all for the advice and consolation. I think it was just a perfect storm of mistakes and a kid wanting to take a bus ride! I'm mostly feeling so awful about it because I'd finally got some confidence about subbing and now it's gone, this is the exact kind of failure I told my therapist I was worried about. Also it's the school my children go to so now feel like I can't even show my face there, and am imagining the admin won't let me come back.