Windfall Elimination Provision

There is something called the "The Windfall Elimination Provision" which may affect you.

The Windfall Elimination Provision primarily affects you if you earned a pension in any job where you did not pay Social Security taxes and you also worked in other jobs long enough to qualify for a Social Security retirement or disability benefit.

See here for details :
http://www.ssa.gov/pubs/10045.html