A lot of restaurants end up throwing out food that would otherwise be perfectly edible.
If you're a polite and patient individual, you might be able to build up some rapport with the owner or the workers. When my friend was in college, he could barely afford to feed himself most of the time. He was friendly with the cashier at a local fast food place and asked if they could set aside any orders that get sent back (due to being wrong) and give them to him instead of tossing them in the garbage. Not everybody will do this for you, but it's worth asking. I think generally store employees and managers are sympathetic enough to not care about what happens to product that was destined for the garbage anyway so they typically agree.