When I was in third grade, I had an argument with a classmate who insisted that "several" meant seven, i.e. if you had several shirts, you had exactly seven shirts. No doubt she got this from the similarity of the two words. I thought myself quite intelligent for being immune to such simplistic reasoning.
Lo and behold, it turns out I'm not. For years I've thought the word "triage", as used in emergencies with mass casualties, referred to the classification of victims into exactly three groups: those requiring immediate attention, those who could wait, and those who were beyond saving. Today, during the monthly training I attend as a member of the volunteer emergency response team at my company, I learned that the word actually comes from French trier, meaning "to sort." And, apparently, it can involve classification into more than three groups. Like my childhood friend, I was misled by what I thought was the prefix "tri-", which turns out not to be a prefix at all.
This misconception seems to be fairly common, as is indicated here.