User blog comment:Waterwecna/Merlin: what happened to you?/@comment-3468142-20121106053000/@comment-4062486-20121106074223

Arthur not remembering this is just one of the many plot holes in Merlin. Even Merlin has managed to do magic under his nose for all those years, so Arthur does not really seem too notice such things. Also, he is not the only one who seems to have forgotten something: Morgana has suddenly forgotten Emrys as well.