It is tempting to ask engineers more than it was estimated for.
“While you are at this, why don’t you add this too?” As if it were for free, doing that extra thing would cost less, since you are already working on that piece.
It is true that adding a new input field while working on a sign-up form costs less now than if add it one month later.
What is not accurate is that it will be free. If the sign-up form was estimated in 4 hours, for 4 fields, adding an extra field would take 5 hours to complete. If it takes less time, then great, but it won’t take 4 hours.
Make sure to ask what you need before the estimate. Asking after, it is the same as redoing the estimate.