> [If] you have effective sales people you will find that they can and will unintentionally sell vapor that cannot be delivered.

This can happen even if the sales people does not mean to sell vapor.

At least in my experience, they seldom have the domain or program knowledge to pick up on nuances in the client's question, leading them to think they correctly answer "yes our program supports that" while in fact the program does not support exactly that.

As a dev I'm not often involved in sales meetings, but every time I have been in a sales meeting I've averted some form of vapor.

The sad part is that most of the times the client is fine with us not having the exact functionality they ask for, but understandably get upset when they only find out about this post-sale.