Annual Loss of Unused funds
The payroll redirection that you choose will be in effect for the entire plan year. Also, according to the IRS, any unused funds in your account at the end of the plan year grace period must automatically be forfeited. This is called the "Use It or Lose It" rule. Estimates on expenses should always be conservative for this reason. In addition, funds in one account cannot be transferred to another account.
To be sure you maximize the tax savings of the Flexible Benefits Plan, you will want to estimate how much money you should redirect back to the Medical Reimbursement and/or Dependent Care Assistance Accounts. A good place to start is the review of any expenses you have already incurred over the last year.