If your actual contributions are greater than your MAC, you have an excess contribution. Excess contributions can result in income tax, additional taxes, and penalties. The effect of excess contributions depends on the type of excess contribution. This chapter discusses excess contributions to your 403(b) account.taxmap/pubs/p571-018.htm#en_us_publink1000129454
To prevent excess contributions, you should figure your MAC at the beginning of each year using a reasonable estimate of compensation. If, at any time during the year, your employment status or your compensation changes, you should refigure your MAC using a revised estimate of compensation.