An important lesson I have learned if you are on the design side of the project.
Make the specifications clear.
Unclear or contradictory specifications just end up leading to having to continually answer request for clarifications, and can also lead to contractors making extra claims for variations because of different interpretations of the specs.
If you refer to standards, make sure to check that the standard you refer to is the current/newest one. If you specify an old standard (easy to do if you are using your organisations standard specifications) you could be specifying that the contractor do the project in a way that does not meet new requirements. This may mean they will claim for extra costs to cover changes to meet the newest standards or that the resultant product doesn’t perform to expected requirements.