If it's a major supermarket they have cameras above each check out so it will show your mistake and everything is logged on the till, you'll probably get called in and have it formally noted what happened sort of like an interview but not super serious just so they have it recorded, maybe you'll have an informal warning or a refresher on your till training etc depending on which supermarket you work for, but you won't be in big trouble at all or thought of as a thief. Everyone makes mistakes and they know that it's normal, especially as you're new. Don't worry you'll be fine, I also work in a supermarket and I've seen it happen before.