If you are seeing an error like that, it needs sourcing to determine what the Mother's actual death date was vs the child's birth date.
Problem can come in when it is a gedcom import or a year is given for mother's birth vs actual birthdate for child.
I have seen several this week where the wrong mother is attached to the profile, which will also cause the error.
Some people just link a person without checking to see if that problem exists 'or' someone updates one of the records with a death date / birth date without checking relatives to see if it caused an error. They would have gotten the same error, but just ignored it, marked it save anyway and walked away. Unfortunately, that doesn't solve the problem. If you look and cannot find sourcing, a message should be left on at least one of the profiles, stating the problem, so that 'hopefully' someone with see the message and check on it.