It's important to candle your eggs at Day 7 and 14 to make sure the chick is developing appropriately. By those points, you should be able to see the developing chick and even movement. If there's no sign of life at 7 weeks, recheck the egg at 14 weeks. Some people dispose at 7 weeks, but this is personal preference. If you have a "bad" egg, toss it. If you continue to incubate it or let your hen sit on it, it will explode. The smell is quite unpleasant and it could contaminate your incubator. Hens will often clean up the mess, but the smell usually takes a day or two to dissipate.