Not spending enough time on the product because the billable hours are paying the bills.
Clients have different expectations when they are paying you by the hour/project than when they are buying your software. You'll have to say no a lot more when selling software.
Providing support to clients with different hardware, software, levels of ability, and time zone differences. Selling B2B may not be as much of a problem.
Focusing on too many markets/industries. In consulting you can rely on the client providing more of the business/domain knowledge and usually they're more willing offer it.