I ran into the next problem today.
On the site of one of my clients there are a number of field groups. Client told me that data submitted to these fields is not always properly stored.
Suppose: He wants to store the names and birthdates of user's children. Because the number of children is variable, I use field groups for this, with a maximum setting of 10. The user fills in the information for three children. After saving this are all three names in the database are stored but not the birth dates? I was able to reproduce the situation.