Can you get in trouble for not reporting secondary food items in fast food places?

That would be a mistake and usually they are forgotten about. If the server intentionally was giving away free stuff, then they could be terminated. You as the customer will be at the most extreme, banned from the restaurant, but that would be rare.

Staff might get in trouble but the customer generally would not. It would likely be thrown away any way because it would no longer be fresh by the time another customer ordered the same item.

That is called theft. It does not matter that it was given to you in error. It was not yours and you did not pay for it. The correct thing you should have done is returned it and said I have been given this by mistake. They may even have said you could keep it. So it was theft and if you consumed it you can add greed to the list.

They have to throw it out if you give it back. The loss is taken into account under a budget for shrink. The manger/franchise would get in trouble if the store's shrink goes over budget by way of him personally taking a lower salary. If it costs $1 per 5oz shot of liquor. and all expenditures are $100,000 with a gross of $150,000, but your bartenders tend to pour 6.71oz shots on average, that can either be given to you by way of pricier drinks or taken from you.

depends on where you are, i would tell them if youre worried about getting in trouble

Yes

No, you can't.

Probably nothing will happen other than you will know you stole from them. (Any time you get something that you know was not intended to be given to you and you fail to return it, you are stealing.)