A mistake on a California tax form could cause thousands of taxpayers to make an expensive error on their state tax returns. The Franchise Tax Board announced Tuesday that an error on a printed California tax form may lead some California taxpayers who file paper returns to understate their home mortgage interest deductions. The typographical error is on early versions of Schedule 540, California Adjustments, which is used to adjust or claim federal itemized deductions. The tax board is rushing to get corrected forms out to the public and to let taxpayers know about the problem.