Via Forbes: Business Plans Gone Wild:

Mistakes are common with business plans.... Google's original business plan did not even include paid search (which, of course, is the core business model).

Common Mistakes:

• Not Hooking The Readers (Financiers) Quickly
• Unrealistic Funding Needs
• Over-Reliance On business plan software

The Business Plans Gone Wild is a really good article and should be read by everyone writing a business plan.