In the presence of dry sulfuric acid at 100 degree c, salicylic acid can react with itself to form a water-insoluble precipitate. how will this side reaction affect the percent yield and purity of the aspirin?

The side reaction will affect the purity and percent yield that, it will decrease the purity and also decrease the present yield.
Due to side reaction, the amount of aspirin which is being produced will be less for some amount of starting material which is used. The present yield will decrease. The insoluble water which is being precipitated is formed as a result of the side-reaction which will remain with aspirin and finally contaminate the aspirin.
The purity of aspirin will also decrease. 

They are all full and don't need any more electrons in there outer shell.