1. Grease, cooking oil, or butter. They end up mixing with other stuff and form a thick sludge that can clog your drains up. Another thing that does it is mayonnaise.
2. Egg shells. Don't jam them in your garbage disposal. They can break the blades. And the tiny shell fragments can stick to stuff and cause clogs.
3. Coffee grounds. They end up sticking to grease and oil that's already in your pipes, so they can make clogs even worse.
4. Pasta. It keeps expanding from all the water. Don't worry if it's just a noodle or two though. The same thing goes for rice.
5. Flour. When you mix it with water, it's basically like glue. So if you spill some on the counter, throw it out. Don't wipe it into the sink.
Source: Food and Wine