If you have experienced a material change in financial circumstances, you may be able to modify your alimony or child support obligation. For example, if you have a job loss, or if your child has reached the age of majority, are common reasons that the court will modify support obligations.
For an example of an extreme change in circumstances, you may want to note the case of Chris McAlister. Mr. McAlister, who formerly played in the NFL for the Baltimore Ravens, has gone from a $55 million dollar contract to being broke and living in his parent's basement.
Hat tip to the Steelers fan who brought this to my attention.