Are you intending to make the software do what you promised clients before you meet with them?

With clients it's the same with patients for doctors.

They never promise you something - for the simple reason that the worst can happen.

Don't promise stuff. Inform, counsel, work out realistic contract, fulfill it as good as you can do.

@IntrusionCM from my experience with doctors, "oh you found a bug in the UI, just give it some time and it'll go away by itself in a few months or so. If it doesn't, come back and we'll review it again". Spoiler: the result of the review is to just give it more time.