Really, it's just an excuse to show off how pedantic we can be! Try to back things up with sources though, if you're expecting people to contest your statements. Here's hoping we can make a few people go, "Huh, didn't know that, cool!"

Here's a few of mine:
The morningstar weapon was not, as almost everyone believes, a weapon made up of a wooden shaft, a chain or rope, and a spiked ball at the end. Rather, http://en.wikipedia.org/wiki/Morning_star_%28weapon%29. The weapon frequently misnamed morningstar is in fact a http://en.wikipedia.org/wiki/Flail_%28weapon%29.
The .38 special calibre is not, as many people believe, a peashooter calibre, being in reality a medium-strength handgun calibre, virtually the same as the standard 9mm used in most pistols, except that it fires a heavier round at lower velocity. It's not a hand cannon, but it's certainly not the spitball shooter most video games and films depict it as, and its +P and +P+ ammunition increase its stopping power significantly.
Duels in the Old West weren't very common, certainly not as common as the movies predict, and contrary to popular myth, duels were not survived by who was fastest on the draw. As any handgun user would tell you, revolvers and pistols require careful aim to have a chance at hitting something. Drawing and spraying the area meant you hit nothing, and the winner was usually the man who took the time to aim.
A Koala bear is not a bear, it's a http://en.wikipedia.org/wiki/Koala.
There is no such thing as a bullet-proof vest. No body armour is impenetrable, which means the correct name is bullet-resistant vest.
Speaking of armour... The idea that armoured knights could not stand up when knocked down in combat or were winched onto their horses is a myth. Plate armour was heavy, sure, but its weight was distributed very evenly across the body, making it less tiring to wear than chain mail, whose dead weight hung from the body, straining the shoulders in particular. Plate armour was actually not that restrictive to movement as many people think, since it wasn't made to withstand blows, but rather to deflect them by angling the metal. http://www.youtube.com/watch?v=xm11yAXeegg is some dude on Youtube proving the point in a slightly embarrassing way. Also as a side note, medieval knights did not do battle in the ostentatious full plate armour often seen in films. Gothic plate wasn't introduced until the Renaissance, and even then it saw little use in actual combat.
You can not catch a sixually transmitted disease from a toilet seat. Pathogens http://www.abc.net.au/health/talkinghealth/factbuster/stories/2011/05/12/3212346.htm, so even if some managed to make it to a toilet seat, they would be dead very quickly. Even if they did survive, they would not be able to infect a human being since they require a point of entry, and typically, those never even touch the seat. So if you have one, just fess up rather than resorting to the toilet seat myth

The fall of the Western Roman Empire was not an enormous bloody battle or a fiery cataclysm. The http://en.wikipedia.org/wiki/Western_Roman_Empire gradually ceased to exist as a geographic and demographic entity by massive people movements and different people settling in the Empire, which had become extremely poorly defended. There were small invasions, to be certain, but the Roman Empire did not 'fall' as such, rather than simply fade. The date of 476AD is set as its marking point because it was the moment when Odoacer the Goth (not the eyeliner-kind) deposed the last Roman (child) Emperor Romulus Augustulus, but that date is simply chosen because they had to pick one determined point in time.
A tomato is not a vegetable but a http://en.wikipedia.org/wiki/Tomato, even though it's often lumped in with the vegetables for culinary purposes.
In cigarettes, the the tar is not the addictive substance, the nicotine is, while it's the tar that is to blame for most of the toxic effects. Many cigarette manufacturers ramp up the nicotine in their cigarettes because it makes them more addictive, and most people are aware of that. But what many people don't know is that many cigarette producers also http://www.ncbi.nlm.nih.gov/pubmed/16904804 to their tobacco to make them even more addictive and make them have a more attractive taste, and to increase the acetaldehyde, which has seriously addictive properties as well.