After completing an experiment, a student finds that the percentage yield exceeded 100%. What could have occurred to the sample to lead to this?
Below is a basic example.  

If percent yield is based on a mass, then a >100% yield can result from a product contains materials that should not be there.  For instance, a product that should be anhydrous may not be completely dry and water would add mass to the product increasing your yield.  Other present contaminants can also do this and can even appear from a product that reacted with the surrounding atmosphere.