In his slides for the PSU Lab2Market program, Jon Summers of the law firm White and Lee identifies the following top-10 mistakes from a legal perspective that startups make:
10. Missing 83(b) Election Deadline (what's that?)
9. Selling Stock to Non-accredited Investors (what's an accredited investor?)
8. Forming Entity as S Corp or LLC instead of C Corp (see "Choosing a Business Structure")
7. Failing to Register Stock Option Plan
6. Waiting too long to Form Entity (see "Protect your Intellectual Property")
5. Failing to Vest Founders Stock (see Buy-Back Provision)
4. Bringing in Tainted People
3. Disclosing w/o a Sufficient non-discloure agreement (more here)
2. Not getting a non-compete signed before an employee starts work
1. Waiting for Funding to Fix Problems (don't wait; fix them!)