(OK, properly speaking, these are homophones – but way back when, my teachers confused me and homonym is the word that pops up first.)

Au fait and ofay

I heard a reporter on the radio just now describe some Mexican kids as au fait (the story was on the anti-emokid violence in Mexico). For a confusing instant, I heard ofay and the sentence made no sense to me. Then my head reoriented – a grin ensued.