In Abbotts Dairies, Inc. v. Philadelphia, 436 Pa. 131, 258 A.2d 634 (1969), the Pennsylvania Supreme Court held that, because of the hardship involved, Philadelphia's mercantile license tax could not be assessed retroactively against companies that had relied on their exemption status for 13 years. Citing equitable principles, the Court agreed that the companies could not be taxed because they had reasonably relied upon exempt status and this would create a serious hardship on them, which would far outweigh any good which might be derived from the assessment. Only after the companies were put on notice could the city assert tax liability.