I frequently get into discussions with friends during which it becomes clear that someone is Totally Wrong about some fact, but no one knows enough to truly disprove them.
For example, canker sores are nasty little sores on the inside of your mouth or on your gums or the inside of your cheeks. They are NOT in fact "cold sores" which are caused by a variation of the Herpes simplex virus and are recurring and on the outside of your lips. Canker sores are caused by a variety of random things, like food allergies/sensitivities, rough toothbrushing, stress, and lack of sleep. The Mayo Clinic (the ultimate source) also recommends switching to toothpaste with no sodium lauryl sulfate, which actually works, I think. Tom's of Maine makes some, but they were pricey so we got Weleda's Salt Toothpaste which is good but makes you thirsty, and Peelu toothpaste which Zeb hates because it has weird fibrous bits in it but I think is not that bad.