cross contamination

I was in Grand Haven Michigan on Sunday around 5pm. My husband and I decided to take our children out to eat. I have to admit the place was clean and the staff seemed to be well dresses and polite. But thats where it went down hill from there. I ordered and whopper without mayo and ketchup, what i got was a whopper with out onion and had everything else on it. So i took it back up to the counter and said this sandwich wasnt right that it had everything on it and no onions, which means i had to open the sandwich to see what was on it. The girl behind the counter said o ya thats the drivethru sandwich and she placed it in the bad and ran to the drive thru and gave that sandwich to that customer. I was just grosses out. I work in a hospital and i know whats out there. If I would of been that customer I would of been very upset. Who knows what I could of done to that hamburger before i took it back up there as in taken a bite and wraped it back up so on and so on. I just wanted to to be aware that has happened and hopefully you can correct it. thanks