During a close approach by Mars in 1877, Italian astronomer Giovanni Schiaparelli fancied he saw thin dark lines on the Martian surface. He called these lines "channels," which in Italian is "canali."
Now, in Italian, "canali" can mean either "channels" or "canals." Somehow, the "canal" version won out, which implied that the lines were an artificial construction. This story might have just become another folk tale, except that the American astronomer Percival Lowell pounced on it, and insisted that the artificial canals were real. Lowell even made up a whole Martian ecology, describing a dying race that had to use vast, planetary canals to carry water from the polar caps to struggling crops. He pointed to the observed surface feature changes on Mars as evidence of crop cycles. None of this was crazy per se, but Lowell's tenacity in the face of contradictory observation DID border on the crackpot...
Lowell was an influential man in astronomy circles, and managed to convince several other astronomers that the canals were real, despite the fact that very few astronomers reported actually seeing them. Indeed, E.E. Barnard, whom many consider to be the sharpest-eyed observer of Lowell's day, never saw them. Lowell eventually mapped dozens of "canals."
When we began to send probes to Mars, it was discovered that there were a few huge surface features, such as the Valles Marineris that did actually correspond to some of Lowell's canals. It is possible that Lowell might have actually seen these during periods of extremely good seeing (and when they were not obscured by dust storms). However, the majority of his canals do not correspond to any fixed surface feature.
The point of all this, and the reason Lowell made my original list of bad science, was that he was not a nut. He was an important and very accomplished astronomer--for example, he correctly predicted the existence of Pluto, and founded the observatory where Clyde Tombaugh discovered that planet in 1930. He just had a bad day...
Of course, the ironic end to the story is that there ARE "canali" on Mars. The planet is covered with what appear to be dry riverbeds. They are clearly of natural origin however, and are far too small to be seen through even the best Earth-bound telescopes. I don't believe even the Hubble telescope has spotted them.