Medical advances have proven a number of past beliefs wrong, though not as universally as you might think, for example while it is acknowledged that blood letting and leeches are useless for curing most ailments there are still certain things they are used for effectively but not many.
Medical beliefs in history have been responsible for the deaths of thousands of people and for others they just made sure that their last few days of life were painful. Someone dying a slow horrible death from an unknown illness would not benefit from being bled or worse having a hole drilled in their head. Drilling holes in the head has been widely practiced; it was called trepanning and would sometimes be used on mentally ill people because they thought the hole would allow the bad spirits to escape. Trepanation is one of the oldest medical practices and trepanned skulls have been found dating back as far as 5,000 B.C. Many people actually survived the process but if they weren´t a bit mental before then drilling random holes in their head would probably make sure they were by the time the "doctors" were finished. It was also supposed to cure migraine headaches and epileptic seizures but of course it didn´t. This century there have been various people who claimed that trepanning leads to higher consciousness. The only legitimate use of trepanation which is still practiced is to relieve pressure on the brain which may have been caused by disease or head trauma.
The idea that the world is flat was not really ridiculous until we got the evidence. It appears in ancient Mesopotamian writing and in classical times many Greeks also wrote about the idea. In 330 B.C. Aristotle provided evidence that it was in fact spherical because travellers going south observed the southern constellations rise higher above the horizon. The argument about the earth being flat raged for many years until we discovered gravity and of course there were always people who believed it was a sphere; it has become a popular idea for us that everyone in the past thought it was flat but that´s not really true.
People used to believe that disease was transmitted by smell. Historically in many cities they built wide avenues to try and stop the smell and therefore the disease from spreading. Since it was actually rubbish and human waste which infected water supplies that spread the disease having wide spaces which weren´t packed with waste actually had a positive effect some of the time, sometimes it had a worse impact as in 1849 in London when a reformer called Edwin Chadwick persuaded authorities to get rid of the bad smell by dumping sewage into the River Thames which was London´s main source of water, an act which resulted in spreading cholera to the masses. Louis Pasteur eventually discovered microbes in 1864.
Alchemy was another popular belief in days gone by and largely involved the idea that you could turn base metals into gold and maybe discover an elixir which would make you immortal. Despite countless people spending their lives in pursuit of these goals alchemy never got results, which is unsurprising since the aims are impossible.
Witchcraft was another ridiculously popular belief and there are even some cultures today who believe in witchcraft as anyone who studies anthropology will know. The African conception of witchcraft is quite different and they believed more often that the person could cause bad things to happen merely by willing it. The idea in the west was that some people, usually women, had mystical powers and could make things happen by concocting spells and potions, in Christianity they believed the powers were gained by striking a deal with the devil. The women would hold black masses where they would dance naked in the woods at night and sleep with the devil and many people claimed to have borne witness to this. There were a number of witch hunts across Europe, the worst period for witch hunts in Scotland was the end of the 16th and beginning of the 17th century. The traditional western methods of testing witchcraft were insane, trial by fire would often involve making the accused walk over hot coals or pluck an object from a boiling cauldron, if they remained uninjured they would be judged innocent, sometimes they would wait a few days and if it had healed would announce that God had intervened and healed them thereby proving their innocence. You would think that since no-one could avoid injury performing such a task that the lack of injury would be proof of their witchcraft but apparently not. Then there was the classic trial by water, if you floated you were a witch, if you sank and drowned you were innocent, either way you were dead.
Most of the really daft historical beliefs fall into the category of superstition which is sadly not something we have managed to shake off and the world of today is still full of people who inflict their idiotic superstitious beliefs on others.