At the end when Jeb swims out next to Nancy, her hair and bonnet are dry. When Jeb then reaches up to dunk Nancy, her hair and bonnet are soaking wet.
Royal wilder was only 10 years older than Almanzo, not the 25 years as depicted. Also, while royal had a daughter, her name was Bernice, not Jenny.
Belt loops on pants were not invented until 1922.
As for Hurricane Harriet's Happenings, the word hurricane was not coined until the 1950s.
Jeb is scared to go swimming with the other kids, but even kids shorter than him appear to just be standing on the bottom of the lake. Even if he couldn't swim he could have gone in with them.