Mint error $20

This is a $20 mint error note that I found one day just before I paid my bills. I studied what it was and found out that it is an inverted corner of a $1 bill taped on at the mint. Someone told me " They don`t do that at the mint, in fact they are not allowed to have tape at the mint, so it must be fake." Well, I took the bill to the bank and they told me that it was a real $20 bill. So it goes back to the fact, how did this bill come to be? Oh, by the way I found out it was clear tape used by holding it up to the light and I can see the tape. My question is, has anyone ever seen one like it or similar to it? Also, is it a rare error because it has two different denominations on it?