The obstacle may be financial in nature, such as unplanned expenses or a failure to achieve initial revenue targets. It may come in the form of a person or a group of people who stand in direct opposition to your plans. You'll know when you encounter one of these unmovable obstacles when you find yourself feeling defeated. It's at this stage that most goals get derailed. I think of these unmovable obstacles as mules.
Mules have three key characteristics.
They will completely stop your progress. They seem to wait until you're moving full steam ahead, and then step out in front of you to block the path to your goal. When you attempt coax them to move, they ignore you. When you try to move them out of the way, they plant their feet and refuse to budge. They don't go away.