When I read recipes involving boiling a soup/dessert and adding sugar, they often say to add the sugar towards the end of the boiling. Why is that? Are there consequences of adding sugar early?
To add onto Walter's answer, adding sugar towards the end for taste when boiling ensures that the sugar dissolves faster so that the results of your addition are more accurate and immediate.
If you added the sugar before boiling, you run the risk of adding too much sugar because you tasted it before all the sugar dissolved and you added more as a result.
There might be a chemical reason for the delay in adding sugar. The properties of sugar change dramatically with exposure to heat, which is how caramel and other candies are made. While caramel usually requires high heat (well above boiling - 300 degrees F and up) over a short period of time, the same chemical reaction can occur at lower tempuratures when the sugar is cooked for a long period of time. This could dramatically affect the flavor of the soup/dessert you are cooking.
An interesting article on sugar/caramel cooking experiments that explains what's going on: http://www.curiouscook.com/site/sugar/