The largest problem is that the people telling you what they want are not good enough to through every possible path the user may want to go down. This leads to specs that are ambiguous at best, contradictory at worst. It takes a good developer to see the hidden things in the spec and raise the questions that need to be asked before the development goes too far down a bad path.

Programmers have it worse: http://ifaq.wap.org/computers/architects.html