One of the biggest things I have learned since I have been in IT is that you have to develop the skill of managing customer expectations (to clarify, the term “customer” means the people for whom you are doing your job - clients, users, etc.). If your customer believes you can perform a service that you cannot, then you have not done a good job in managing expectations, and you will likely end up dissapointing him and hurting the professional relationship.
From the sales POV, if a customer believes that a certain product can perform functions that it cannot, then the customer’s expectations have not been managed. The customer has to know what a product is capable of and how it will fit and perform in his network. If this is not fully explained, then the sale can turn into a disaster.
This is a hard thing to do when it comes to sales since customers often do research when looking into a solution, and the marketing departments of companies typically make it sound like their product can cure world hunger and make you a sandwich at the same time it is keeping your network totally secure (and it does all that in a nice little 1U appliance that takes five minutes to install and configure). And whether we like it or not, customers will often believe the claims because they want the claims to be true. They need a widget that will cure their ills, and many are short-sighted enough to try to find that widget.
So as the sales person and sales engineer (often the same person), it is imperative that the expectations for a product are managed up front. If the customer calls you in and says that the brochure for XYZ Security Widget says that it can perform a certain function, you have to be able to explain if the claim is true or not. You have to make it clear that often case studies are done in pristine situations. And you also have to clarify that the “setup” of the widget (yes, the one that takes only five minutes) in a network often means that it was simply screwed into a rack, plugged into the network, and assigned an IP address. There is usually little to no configuration done on the widget, and it is absolutely worthless in this state. You have to enlighten the naïve customer by telling him that trade rag product reviews are often rigged (it sucks, but it is true). You have to do all of this because you want to maintain the customer as a customer.
You also have to elucidate and educate because you will be trying to sell professional services to install the widget for the customer, and they are going to balk big time when your statement of work says 40 hours instead of five minutes. And they are going to balk again when you try to sell a training class that takes 4 days and costs $2000 a head.
So if you want to keep your customers, manage their expectations. Make sure they know what the real deal. You will help them avoid many unpleasent situations (also, be sure to let them know, in a non-braggy way, what unpleasant situations you helped them avoid 
- they will appreciate it more).
Vet
