I agree with Jeffro.
I think it is highly unlikely you have a mint error. The SBA dollars were minted in '79, '80, '81 and again in '99. The Sac dollars started production in '00. It is not likely a Sac planchet could have made it in the bin during SBA production. (Sure, they existed in '99 for test strikes but they were carefully accounted for.)
The two coins weigh the same but they are composed of different materials. If you really think you have an error coin you could ask a reputable coin dealer to look at it or send it to ANACS or a grading service to authenticate it ($$$).