What is one character flaw Bill or Sam has in "The Ransom of Red Chief" by O. Henry?

Both Bill and Sam assume too much. First, they assume that all the people of the small town of Summit are "undeleterious." This means they assumed the people were harmless. Bill and Sam also assumed "philoprogenitiveness" (the love of one's children) is strong in small towns such as Summit. From their research on Ebenezer Dorset, Bill and Sam assume he would break down immediately and give up whatever ransom they demanded for his son (Red Chief/Johnny). This makes Sam and Bill presumptuous. They also show some arrogance in supposing that everyone from small towns behaves the same way. Their major flaw is assumptions. One might add that Bill is weak and/or has no babysitting skills. If Johnny is the terror that Sam describes, though, this is not really Bill's fault or flaw. 

Sam and Bill's assumption of the town's "philoprogenitiveness" comes back to bite them when they demand the ransom. The first ironic twist that counters their assumptions is that Johnny is too much for them to handle. Johnny has the time of his life being kidnapped. The second twist is when Johnny's father makes them a counteroffer and demands Sam and Bill pay him to take Johnny back. This counteroffer disproves the con men's notions that people in small towns are simpleminded and love their children absolutely.