Sometimes fast food items are bad because of the way they're handled or kept either before or after being made. A thread on Reddit focused specifically on this. Workers from chains like McDonald's and Burger King all the way to Taco Bell and Domino's chimed in to warn unsuspecting patrons of the gross fast food practices each restaurant took part in. This is apparently a problem that spans fast food chains of all kinds, though you'll notice a few key repeat offenders on this list.
The workers specifically talked about what exactly a customer should never order from their restaurant. Either because of gross practices, never-cleaned machines, bad ingredients, or just general unhealthiness, most places always have at least one thing that a customer should never order.
Warning: this will alter views on the globes popular fast food chains. Be prepared to be grossed out.