This is a link to a blogpost at Miramar College's (San Diego, CA) English Center blog site from a workshop held there last year (
Grammar Mistakes from Workshop at Miramar College). It contains some entertaining photos illustrating common mistakes in spelling which reflect poor English grammar. Because spelling and pronunciation are not closely linked in English, it is easy to make these mistakes. A spellchecker won't necessarily catch these errors because the mistaken words are correctly spelled, but misused in context.