Generally, I'd say it is a great thing to have another family member who had a head start doing research, and hands you a whole pile of research and documents. The downside is that when obtaining a large pile of documents at one time, I did not scrutinize them with the same eye for detail that I might have if I had done the research myself and had to scrap for each piece of it.
This past week I was looking through the sources for the Irish branch of my family tree, hoping to find some glint of info that might help me to make a connection with some of the 23andme Relative Finder cousins that I have been in touch with. In particular I was looking at my great-great grandparents Norris Donaldson and Eliza Reid, who were both born in Ireland and settled in England. The only parent I had on my tree was the the father of Norris, Alexander Donaldson. He also moved from Ireland to England, so while I had plenty of sources for both men, I could not find anything that actually showed that Alexander was the father of Norris.
I began to search my paper and electronic records to find the missing link. I finally was able to locate it when I viewed the marriage certificate of Norris and Eliza. English marriage certificates of this time period only list the father of each participant and here was my paper trail to show that Alexander was the father of Norris.
But, what I had failed to see when I obtained this document from my cousin last year, is that Eliza's father, James Reid, is also listed. I am no closer to having any idea where in Ireland any of these people lived, or where I might even start to search for more info, but I did discover the name of one of my 3x great-grandparents, and only the sixth one on my family tree.