I remembered my old post and I decided to put them in a list.

Words AFFers commonly mix up 

Foreword vs Foreward (jessandheraddiction)

Principle vs. Principal (yuu014)

Definitely vs. Defiantly (I see this ALL the time)

To vs. Too

Your vs. You're

:: Kudos to those who know the difference between these words! ^0^v

:: Here is a longer list: Commonly Confused Words - Oxford Dictionaries Online

Common misspelled words in AFF

Grammar vs. Grammer

Prologue vs. Prolog/Prolouge (conchobar)

:: Kudos to those who identified the correct spelling ^0^v

:: Check this out: 100 Most Often Mispelled Misspelled Words

You can add more to this :)