While in some cases an express instruction to the jury to disregard testimony injuriously admitted is properly held to cure the error, yet the courts are cautious in the application of this rule. It is not an easy task to unring a bell, nor to remove from the mind an impression once firmly imprinted there, and the withdrawal of the testimony should be so emphatic as to leave no doubt in the mind of the juror as to the unequivocal repudiation by the court of the erroneously admitted matter, and even then, in a case where the testimony is evenly balanced or contradictory, courts hesitate to sanction such withdrawal, though it seems absolutely necessary to permit this course in some instances.