I have had property owners be a big hindrance towards safety (allowing their employees to circumvent our safety rules); architects who expect the impossible (I haven't had too many problems with engineers, once you get them in the field you can show them the error of their ways:)). My biggest peeve is project managers with little or no field experience.

I don’t know if there is one answer. I have had problems with government engineers and getting them to agree on the best way to complete a project. I have even had where 2 government engineers working for the same consulting company couldn’t agree on a plan that we had written.

But that being said I have also had many problems with property owners, who think the know everything.