What should one do if HR made a 'mistake' with salary during interview process?

I was just offered a position after several months of interviewing. The HR people apparently made a mistake with the monetary figures when discussing salary during the interview process. When I was offered the job they announced that the position is about 10% less in salary than they initially told me. The position is really good but I feel a bit cheated. I told them I would think about the offer. What should I do?