1. Not getting money up front
2. Extending credit
3. Open ended meetings
4. Being insufficiently explicit about what is being delivered
5. Working on a handshake (seldom a problem with big clients actually) - get them to sign something
6. Not actually meeting in person at least once
7. Not being clear on who owns the code/technology (if you're going to do more or less the same thing for the person across the street then make sure to let the client know that they are getting a license (or something similar))
8. Scheduling meetings in their downtime, but your worktime
9. Not having a template, or even an idea of what a good referral would look like
10. Having a specified finish line - much more important for the smaller client than the larger ones IME