> So what exactly constitutes an error then?

It's standard contract law. Every time you purchase anything, you're making a legally binding contract, even buying a sandwich at a shop. Under centuries old contract law, there must be a "meeting of the minds", both parties must understand what they're getting into. If one person makes a obvious mistake (e.g., pricing gas at $0.02/gallon), there is clearly no meeting of the minds, the contract is void, and the seller can ask the buyer for their money back.

> Why would an employee expect different treatment if they believe the laws are just?

Because they don't know the law, and they think they're brilliant and the first person to come up with such a scheme.

Are you a company? I mean come on, let's be realistic. If you accidentally pay a company money and ask for it back it goes through a reasonable process to assess your request. You probably start by going to customer service, CS then refers it to finance, finance has to identify the transaction, check that it's a genuine mistake, has to account for that in their systems, authorize a refund, and pay the money. Each step in that probably involves a different person. So yes, it takes time. If a company pays you money you go through the same process. The difference is that because you're not operating on a large scale all of those steps can be done by you, very quickly. If you were receiving tens of thousands of payments per day, all from different people for different reasons then we'd expect you to take the same amount of time to refund. It's not that companies are necessarily treated differently, it's that they're treated in a way that is equal, but not the same.