The statement that is true of a flexible spending account (FSA) but not a health savings account (HSA) is:
If you don’t spend it within a certain period of time, part or all of it reverts to your employer.
FSAs typically have a "use it or lose it" policy, meaning that funds not used within the plan year (or a grace period, if applicable) may be forfeited, while HSAs allow the account holder to keep the funds indefinitely.