Is there something you think you know but for which you have no real proof other than you have always believed it?
While it doesn't have direct genealogical bearing, I have been reading "Pillars of the Republic" by Carl F. Kaestle. One thing I learned while reading the book is that there were many schools in the 1820s-1840s that enrolled children as young as 4. This trend changed in the mid-eighteenth century. I just always assumed that there was no schooling at all for children that young until the 20th century. I never read that anywhere, I just assumed it.
Is there some "fact" in your genealogy research that you never read, never heard, but just assumed? And is it causing that brick wall in your research?