Kathleen was noticing that the 1940 census had her father 20 years older than he really was.
The lesson, when you find a census it is often a great resource, however, as a stand alone documentation it may have some incorrect info.
I love finding censuses on families I am researching. Yesterday I found the family of a woman who was a child we knew of parents we knew of. Through British census info we learned she ran a boarding house, that she outlived 4 of her 5 children. We previously didn't know even if she was married. We found grandchildren. We found out her married name. We still need to find her husbands name as she shows up in a census as a widow, at age 33.
Yes you can glean a lot from a census. One thing I like about Ancestry.com is how it suggests other census to review on the same page of the particular census you are looking at.