You can also get an incorrect percent yield of over 100%. Many chemical reactions of course occur in aqueous solutions. Therefore when you are measuring the final mass of product, you often fail to heat off all the water and you think you have a pure crystalline product but there is actually water still trapped in there. Therefore you can get a percent yield of 102%.
Why would this be worse then a percent yield of 98%? They are both off by 2%?