After you've tried to save, scroll up to the top. You'll see:
Warning: Check the data.
A birth date (Whittemore-404 born 12 Jun 1787) should not be within eight months of a sibling's birth date (Whittemore-405 born 2 Dec 1787) unless they are twins born within a day of each other.
You can correct the data if it's in error, or click the Save Anyway button to override the warning.