At the beginning of 2009, you should refigure your 2008 MAC based on your actual compensation for 2008. This will allow you to determine if the amount that has been contributed to your 403(b) account for 2008 has exceeded the allowable limits. In some cases, this will allow you to avoid penalties and additional taxes. See chapter 7.
Generally, you should figure your MAC for the current year at the beginning of each tax year using a conservative estimate of your compensation. If your compensation changes during the year, you should refigure your MAC based on a revised conservative estimate. By doing this, you will be able to determine if contributions to your 403(b) account can be increased or should be decreased for the year.