We bought a house. It was just what we wanted and our realtor assisted in finding it and successfully negotiated the deal. We had the house inspected and the mortgage company is fine with everything. The house is on MLS as 2000 square feet but in fact is about 1800.
We don't really care as the house fills our needs - is there something wrong with this?

It was most likely appraised and a good appraiser would have mentioned the discrepancy and it would have certainly impacted the valuation, but it didn't kill the deal.
If I like the property I would forget about the 200 sq. feet. You could use it for renegotiation but if you have already signed a contract, you would have to consult an attorney for advce. I am not a real estate expert.

I had a similar situation - bought a house that was listed as being on 3.7 acres, but in fact was only 3. We didn't find out about the discrepancy until two days before closing, after the property had been surveyed, appraised, mortgage secured etc. At that point we could have tried to negotiate for a lower price, or backed out of the deal I suppose, but we decided to go ahead with closing and try to get something back after the purchase (which didn't work). In your case, you are happy and the bank is happy, so no harm. However, be sure to check with the tax assesor in your town to ensure that your property taxes are based on 1800 sq ft ,not 2000.