When dancing with the ancestors, one of the biggest things that happen are that people assume things. Word of advice: assume nothing!
Example: you're doing research, find the will of an ancestor and it doesn't mention their child, your 5 x great grandmother! So, you assume your 5 x great grandmother wasn't the child of said ancestor whose last will and testament you found.
I have found last wills and testaments that list all the known children of an ancestor.
I have found last wills and testaments that list only the male children of an ancestor.
I have found last wills and testaments that only list the wife of an ancestor.
In the first instance, it's a woo-hoo moment because the last will of 5 x Great Grandpa lists all of his children. Yay for me!
In the second instance, it's a holy crap moment because I'm researching 4 x Great Grandma, daughter of 5 x Great Grandpa, and he only mentioned his six sons and not his five daughters.
In the third instance, well, you can figure that one out.
The main conclusion NOT to draw on Scenario two and three: my 4 x Great Grandma is obviously not the child of said 5 x Great Grandpa because he didn't mention her in his will!!
DO NOT DRAW THIS CONCLUSION. DO NOT ASSUME ANYTHING!
In many cases, when children back in the day married, the father would give them land and/or money and then not mention them in the will because they already gave them land and/or money at the time of marriage.
Or, as in Scenario Three - 5 x Great Grandpa left everything to the wife and made some reference "to be evenly distributed among my children at the time of her passing". Yes, I've run across this as well. It's a family researcher's nightmare: the will doesn't name the children. It doesn't mean the children did not exist, or that I'm on the wrong trail, only that 5 x Great Grandpa didn't bother to name the children in the will. It happens!!
So, when dancing with the ancestors and you come across a last will and testament that doesn't mention the ancestor you're researching, don't assume you're on the wrong trail, you have the parents of said ancestor wrong or anything along those lines. Instead, take a step back and consider that maybe the ancestor you're researching already received a bequest from the parent. Try to hunt down land records or other records that can prove the parentage, rather than simply - as I've seen many people do - that because the ancestor is not mentioned in the will they don't exist or you have an error in your research.