UPS, FedEx: What if you drop them in wrong box?

What happens if you put a pre-labeled UPS package into a FedEx drop box? Do they transport it to FedEx for you? Throw it away?

Is there some sort of agreement between USPS, FedEx and UPS that they will correct these mistakes and drop them at the correct carrier?

That happens all the time, or did when I worked for FedEx. The two companies generally have by boxes near one another, particularly in big city office buildings, so our guys, upon noticing such an error, would just chuck a UPS package in the first appropriate by box they saw.

What did they do when someone threw a FedEx Ground package into a FedEx Express box? (To the best of my knowledge, Ground doesn’t have any boxes.)

I imagine they take it back to the customer or to a FedEx Office (formerly Kinko’s). Taking it to the Ground facility wasn’t practical; it was too far from the Express hub).