Not getting a skilled evaluator to write the evaluation section for all grants that require evaluation. A big no-no. Waiting until the last minute to get critical documents signed. Always get as many as possible of the certifications and other required forms signed ahead of time. Never wait until the last minute.
People do take unscheduled vacations and they do get sick.
Developing a budget that does not complement the proposed project. Isolate the proposed project. Lay it out on paper and identify the costs associated with each element. Waiting until final hour to xerox the application. Complete all xeroxing a day before you mail.