I always used to notice that on t.v. myself. I wondered why they never showed anyone slipping up in their speech or having misunderstandings. But really, if people went around making mistakes like that on t.v. or even in literature, I think it would really kill any sense of tension and drama. I can understand an explicitly ignorant character might forget the name of someone important, but peppering content with little mistakes just to be realistic would probably bug me more than their absence.