We make them constantly in our research whether we realize it or not.
And they can negatively impact our research, especially when we do not realize it.
Recently I discovered a miinor assumption I made--nothing major. The assumption did not hinder my research and the discovery that I made it did not cause me to have to re-evaluate weeks of work. It simply served to remind me of the importance of having sources for not just "facts" about our ancestors, but also for their potential motivations as well.
A recent blog post discussed a letter that was in my ancestor's Civil War widow's pension file. Written by her Congressman, it was in support of my ancestor getting an increase in her pension in response to a bill passed by Congress in 1916. I assumed that letters of this type were unusual, until I discovered that pensioners were told to contact their Congressman directly instead of hiring a lawyer or contacting the pension office.
My vision of the ancestor immediately fighting for her pension increase and uniquely asserting her rights was dashed. She was simply following orders.
The assumption I made was minor and really only impacted my own mental image of my ancestor.
How many times are assumptions much bigger than that and how many times do they actually hinder our research?