I am lucky in that I had 3 relatives very interested in genealogy, one of whom has since passed away, who created extensive records and shared. The one who has passed away made several trips back to Holland to research records and being as he was from Canada, served in WWII and that there is such a close and warm connection between the Dutch and Canadians stemming from the liberation of Holland, he had people there who couldn't do enough to help him with his research.
For many cultures family names are certainly repeated over and over. It was helpful to know that first son born was named after the father's father, second after the mother's father, etc. Both my mother's grandparents came from families of more than a dozen children. Not only that but a number of siblings from each of their families married each other so just imagine how many kids ended up with the exact same first and last name. I know for both my mother and her brother they were identified by given name and then father's name to differentiate. (E.G. Mary, Uncle John or Peter, Uncle Jake. The girls were able to escape this a bit once they married and took on their husbands' last names. Looking at records their middle names could be an important clue, but back in Holland that wasn't a great help because often they had no middle names. It would appear at one point the government charged extra to register middle names so only the rich paid for that privilege. Once the family was in Canada they went nuts with middle names so many have at least 2. The other thing that happened is that many were known all their lives by a unique variation of their legal name, or even just by a nickname that stuck and bares little resemblance to their real name. (One of my dad's brothers hated his given name so much that he actually adopted the nick name of an uncle, which is used amongst family members, but was mostly known by a different nickname in his work life. It's to the point that when I introduce him to outsiders I let him decide which name to use
Some of my grandparents siblings did not come with their parents. In my grandmother's case some of the oldest were already established and didn't want to come. In my grandfather's case it was actually the oldest who emigrated first and brought most of the rest of the family over later, including parents. However one sister was refused passage when medical screening determined her to be ineligible at the last moment. Since everything had already been sold up and all their money spent on passage for the whole family, they hadn't much choice but to continue so she was left behind with other family. I can't imagine the shock they were all in over that turn of events.
Another reason dates can be off is simply that many home births were not registered right away. I think my mother told me she was 6 months old before she was documented. I am sure the further you go back, or the more rustic the local, the more often that type of thing happened. I can see how the registration date and birth date could become confused. It is also my understanding that in some Catholic cultures it was more important to "assign" a Saint to a child and often they would celebrate the Saint's day instead of the child's actual birth date. Does anyone know if that is common?
I also find it a bit amusing that the lady who stopped researching when she uncovered the lack of a formal marriage in the family's past. Depending where they lived and how far back you go, common law marriages were very common. Sometimes only the more well to do formalized their arrangement.