If I'm not very much mistaken, you would put 'everyones' with the apostrophe at the end of the word, i.e.: "Everyones' spelling mistakes" if the word, in this case 'everyone' is not singular. Quote from englishplus.com : "To make a plural noun possessive, simply add an apostrophe to the word. If the plural does not end in an s, then add an apostrophe plus s." In some cases 'everyone' takes the singular form, i.e.: "everyone is great" but would it not take the plural form here, could the spelling bee divulge please? It is causing me great discomfort