What are the most common legal mistakes first time entrepreneurs make?

In no particular order, common mistakes include:

- engaging, and paying, product and software developers based on an RFP or otherwise without a contract containing appropriate IP assignment, representation / warranties, and licensing provisions, among others;
- not having a trademark clearance search performed before either applying for a registration or investing in marketing with an unqualified mark;
- merely filing Articles of Organization to create a LLC and not having an Operating Agreement drafted by and among the Members;
- forming a Delaware domestic entity because the Certificate of Formation filing fee is relatively cheap, or because forming a Delaware domestic entity is commercially cool;
- using contracts obtained online and without review, whether from LegalZoom or otherwise;
- treating independent contractors like they were employees; and
- spending $5k, $10k, or even more on "invent help" companies which merely provide some basic market research, a thin prior art search, and suggest you go ahead and contact a patent attorney anyway to look at theirs and other search results under Section 103.

Don't do these things, and remember that IP attorneys are here to help you avoid problems.

Adding to Adam's excellent list... A common mistake I see all the time: showing up to a trade show (or elsewhere), making a public disclosure of your product/invention, and not having at least a provisional* patent application filed. Not only does this create risk under under the new regime of first-to-file at the USPTO, but such a disclosure most likely precludes your ability to file a patent application in most foreign countries since they do not have a 12-month grace period as in the US. *In most cases it may be preferable to have a non-provisional filed instead of a provisional.