Customers are not always right, but I'd never tell them that. Unfortunately, in every business, we encounter customers who make unreasonable requests or have amplified expectations. We'd love to say “no” but our yes mentality keeps us from doing it.
Do we tell them “I'm sorry, we don't do that”? Do we agree to do it?
The conundrum is: if we agree to meet their expectations, we could be setting a dangerous and costly precedent. If we don't meet their expectations, we could lose a customer.
Although you have to deal with that on a case by case basis, the best thing to do is to manage your customer's expectations by helping them to understand what to expect. If you have a complex sales process (for example, due to regulations), walk them through it step by step. If you get a lot of requests for something that you can't do, tell them up front what you can do, and leave it at that.
You'll always get unreasonable requests, but by managing your customer's expectations, you should be able to minimize those unreasonable requests substantially.