Without a doubt, one of the most common issues I see pertains to the dishwasher installation. The issue is that the dishwasher drain line does not make a high loop underneath the kitchen sink. In the diagram below, you can see that a high loop is just what it sounds like.
As you see, the dishwasher drain should make a high loop up to the bottom side of the countertop. The high loop is necessary for 2 reasons. The first is to ensure proper drainage of waste water during the drain cycle and prevent siphoning. The second is to prevent contaminated water from the garbage disposal or sink drain from back-flowing into the dishwasher.
An alternative to the high loop would be an air gap that is installed through the countertop. An air gap device is used to create a siphon break for the same purposes as the high loop. As you can see in the photo above, it is a bit unsightly. For this reason most homeowners opt for the high loop under the counter.
Although all new dishwashers come with a high loop integrated into the unit itself, every installation manual I have read requires the high loop be installed under the sink as well. Luckily this is not a hard requirement to satisfy. A simple and inexpensive fix would require a piece of metal (or plastic) strapping and a screw. Both can be found at your local hardware store.
