It just ringed with me. It is amazing, how often it happens with non-experienced users: When they explain specifications, they explain common cases, but forgot to mention any irregularities, saying these cases are very rare and they did not want to confuse you more. As it happens, these irregular cases might cause complete redesign to meet these "hidden" requirement.

I read somewhere that if God, when creating universe, will ask for sign-on on complete requirements, we will be still waiting to implement.

Author proposed "Genesis approach" (RAD): To create something simple in just 10% of available time, covering maybe only 20% of needed functionality, get feedback from customers, and let it grow. This approach assumes you have ready-made tools and tested procedures ready "to hit the road".

Still, IMHO you may consider this especially in your case, when you are not sure about tools to use. Explain your customer that it might be it may take longer to implement full 100% of what s/he has in his mind, but you cannot read his/her mind, and both you and s/he will have better feeling what needs to be done, what are priorities and what features are not feasible.

Of course, if you can read minds, just go ahead and do it.

pmas