One of the biggest mistakes that salespeople make is that they promise more than they can deliver.
So when they're going to close, they think they need to keep promising things in order to make the client buy.
Now, what's the problem with that?
First of all, the client is unhappy, because you promised more than you could deliver, and you end up not delivering what you promised.
Thus, they don't refer you more clients...
Because they don't refer you more clients, you not only make less sales, but also on the sale (where you promised more than you can deliver), you make less profit, because you need to figure out more ways to deliver more than you can afford to deliver!!
And it gets into a mess, and a dwindling spiral.