In 1870 a man proved that the Earth was round. He wasn't the first to prove it, and he wasn't even proving anything out of the mainstream. But that didn't keep a fanatic from spending the next 20 years ruining his life.
The Bedford Level Experiment
Alfred Russel Wallace spent much of the first half of the 1800s making a modest living as a surveyor. His income did not cover his many scientific pursuits (at which he showed great talent) or his few financial pursuits (at which he showed no talent at all). When he saw an announcement in Zetetic Astronomy pledging 500 pounds to the person who could prove the world was round, he thought it was easy money. The term "zetetic" is derived from a Greek word and means, roughly, "I will find out for myself." Finding out for oneself is great in theory. In practice, when people try to find things out for themselves they often make mistakes without realizing it.
When Wallace looked into the announcement in detail, he saw that one contributor to the publication, who went by the name of "Parallax," was making a lot of mistakes. The Old Bedford River had a six-mile drainage canal marked at each end by a bridge. The canal was so long and straight that, if the world were round, a boat at one end would not be visible to a boat on the other end. They would each be hidden from each other by the curve of the Earth. Parallax — whose real name was Samuel Rowbotham — often took people out on the water and showed them the boats at the other end of the canal. The world, therefore, was flat. Wallace had enough experience with surveying to know that density gradients in air, which are common just above water, can bend light back down towards the ground. Parallax and his guests were seeing a mirage.
If Rowbotham had been the one offering the money, Wallace might have claimed his prize without a problem. It didn't take Wallace long to come up with a better way of showing the curvature of the Earth. He would put a series of disks up on poles along the water. He would then stand on one bridge, and look back at the disks. If he was right, the disks towards the middle of the canal would look slightly higher than the rest of the disks, and the disk at the far end would look slightly lower. He would basically be putting dots along the canal that traced the curve of the Earth. When Wallace set the whole thing up he, his flat-Earth opponent, and their agreed-upon referee did see exactly what Wallace expected to see. The middle disk was slightly higher, and the far disk was slightly lower.
Sadly, it was not Rowbotham offering the cash. Instead, it was a nasty little psycho called John Hampden. Hampden saw what everyone else saw, but he also saw something else. He saw cross hairs on the telescope that the three were using to look at the disks. He saw that the middle disk was a certain distance below the cross hairs. And he saw that the far disk was that same distance below the middle signal. (So if the middle disk were a quarter inch below the cross hairs, the far disk was a quarter inch below the middle disk, and a half inch below the cross hairs.) For some reason, Hampden decided that this must mean that the disks were in a straight line, whether one appeared higher than the other or not. No one could convince him otherwise, including the referee, who consulted the actual makers of the telescope and surveying equipment just to make sure that Hampden's observation wasn't relevant. Not only did Hampden not accept the decision, he embarked on a 21-year campaign of harassment, threats, and libel that ruined Wallace's life.
The Fanatic's Revenge
First, Wallace was obliged to give back the money. Hampden, despite issuing the bet in the first place, took him to court and claimed that two people were not qualified to settle whether or not the world was round or flat. Hampden then started publishing insulting letters in publications. When he moved on to death threats, which he charmingly sent to Wallace's wife, he was put in prison. (Note to nutters: If you write, "If your infernal thief of a husband is brought home... with every bone in his head smashed to a pulp, you will know the reason," don't sign it with your own name.) Eventually the harassment became a cycle. Hampden would publish libelous claims and send Wallace threats. Wallace would take Hampden to court and Hampden would be forced to recant, briefly imprisoned, and barred from writing anything about Wallace for a few months. The day after the months were up, he'd be back at it.
It only ended with Hampden's death. His fellow flat-Earthers never changed their mind about his position, although to be fair some changed their mind in regards to Hampden's character. Wallace spent two decades fighting libelous claims and accruing court costs, and nearly went bankrupt. He even got some flack from the legitimate scientific community. A globular Earth was the mainstream view; few people had ever espoused the flat-Earth philosophy. Scientists felt like Wallace's acceptance of the challenge provided publicity for the fanatics.
There are still flat-Earthers today. There are any number of deniers, conspiracy theorists, and generalized nuts today. While, much of the time, a person who jumps into the fray and debates a subject honestly is a hero — there is plenty of history to show us that steering clear can be the wiser course. Being right, sometimes, isn't enough.