Saying that there are exceptions to a general rule only implies that the general rule really is true most of the time.

A software developer often has to manage customer expectations.

If your customer expects you to develop something that is good and cheap and can be developed quickly then you already have a customer expectation problem. I think it’s probably a mistake to just assure a customer that you can deliver on his expectations.

I think the triangle analogy is a good tool. You need to think of a nice way to tell the customer (or the boss), “There is good, fast and cheap. Pick two”.