At this level, there shouldn't be this type of error. Should be, as Rush says, double sourced and triple checked. Whomever that 'one person' is at the accounting firm should have gone over these envelopes many times. Letting two envelopes go that were exactly the same is highly suspect.
You are correct. An error of this sort should never happen at such a high profile event. I just saw on the LA Times website that there are always two cards - one for both sides of the stage. I have not read the story yet, but they have several articles on how it happened. I went there since I assumed LA Times knows how to cover their local industry. However it happened, Steve Harvey is probably a bit relieved at the moment.