It may be a cliche, but one of the biggest mistakes when re-roofing is being "penny-wise" and "pound-foolish". "Saving" a few hundred dollars up front often cost homeowners thousands down the road. This doesn’t include the hassles and potential damage that come from leaking or improperly designed roofing systems.
Take attic ventalition for example - according to industry reports, the majority of homeowners find existing attic ventilation non-existent or inadequate when inspecting a home before re-roofing and that can mean high utility bills. Poor indoor air quality and possible mold growth, as well as structual damage. Your new roof will also wear out quicker if ventilation is not a priority.