Underbidding--intentionally or not--is a common mistake, and it's a really tempting mistake to make, since you want to snag business. However, if you underbid, you'll end up losing money, and can also sour client relationships. What's more, once you realize that you have to complete a project with essentially non-billable time, your motivation can plummet, making it even more difficult to complete the task when it's competing against billable projects for your time. And having these kinds of outstanding non-billable projects creates stress, since you want to procrastinate working on them and yet know you need to finish them. It's a big trap I've seen consultants fall into--myself included.

Greg Miliates
StartMyConsultingBusiness dot com