The biggest pitfall is that you will be successful as a consultant long before your startup will make any money and the temptation to drop the startup and concentrate on full-blown consultancy will be nearly impossible to resist.

If you are going to do this be rigorous from day one and set aside a number of days per week when you do not allow yourself to work on your consultancy stuff at all.

Stay away from consultancy jobs that are even remotely related to the startup you're doing.