If you miss any segment, the remainder of your itinerary will be canceled. If it's the last segment you're missing, it's generally no problem. Missing the first, however, is another story unless you're on a fully refundable fare or you arrive within the "flat tire" window, about 2 hours I think. Depending on your fare rules, you will often simply lose the entire value of your ticket and have to buy a new one at the walkup price. Not a good idea.
Michael