I have done some work as a consultant for a few companies. The first few dives into the foray had a steep learning curve. And luck is your best friend during these times. But most of the time you crash and burn. And you crash and burn mostly because of the following reasons:
Take the path of least resistance
To make things easier, it is human nature to find the path of least resistance. In other words, taking shortcuts to get what needs to be done. Shortcuts make you look bad, because most of the time, they are temporary fixes. In the long run, you end up plugging holes for free.
Be a Cassanova
Engaging in flirtatious actvities with employees is very unprofessional. It gets in the way.
Politics and religion
Do not participate in these discussions. They bring bad juju to your project.
Acting high and mighty
A consultant was hired to do a specific job. Like any other employee. But still considered an outsider. No matter what. Puffing up one's breast feathers just ruffles other's feathers. You are not a god. you may end up not listening to the employees, which are the most important link to gettting the job done. They are the frontliners.
Cutting down other consultant's work
Very unprofessional. Makes you look like a schmuck. To clients and other consultants
Have I made myself clear? A consultant's competition will be thankful after you fall into any of these pitfalls.