As a general rule of thumb, if something is not as it should be, then it is an error, freak or oddity. EFO's are the result of an accident of some kind during the production process. It is a unusual aspect of stamp collecting that an error may be worth considerably more than the perfectly normal stamp. EFO's are valuable because there are so few of them and they are regarded as collectable. EFO's are the speices of a stamp collection. They break the monotony of an otherwise straight collection and add unusual and at times intriguing colours to it.