Preconceptions - theirs or yours
It is very easy to be swayed by preconceptions. The clients may have seen something that they like and want the same for themselves, or something along the same lines. You might have this great idea you have been dying to use for ages, and this could be the big chance ... No, avoid preconceptions. No two problems are exactly the same. Unique problems require unique solutions. The trick is to identify what is unique about this particular problem. Dig for it. Isolate it. Stand it naked in the sunlight.
Do that, and you are half way there!