Percent yield is simply the actual yield (the mass of resultant) divided by the theoretical yield (the most that can be attained). Therefore, the possibility of having a percent yield greater than 100 is impossible unless an error is made during the procedure.
Theoretical yield remains in the denominator while the actual mass of your product stays in the numerator. This can be thought of as "how much you attained" divided by "how much you could have attained." This thought process may help to point out an error in the equation that results in a percent yield greater than 100.