Here's something I got from the cajuninjector.com website:
Too much smoke, or inadequate air flow, can cause a bitter flavor in the products being smoked as can inadequate knowledge of the wood chosen for smoking. If meat receives too much smoke, it will impart a very bitter creosote taste that also may numb your tongue. Inadequate air flow or too strong a wood flavor for what is being smoked, can also create bitter flavors. With smoke as with spices, it is better to use too little than too much. While there is still debate among some grill masters, most cooks believe that smoky flavor obtained after the first two hours is enough that further smoke is a waste of good wood and may well result in a creosote flavor.