In my book, if a stamp is not perfectly printed off or has any type of error than it is a collectible. The printer will catch it one way or another. If the stamps get out to circulation than all the better for the collector. Some of the errors will be valuable and that depends on a lot of factors. Look at the U.S. air mail with the upside down plane. (Sorry U.S. collectors. Forgot just the exact name for that stamp.)
I know that it's for the thrill of seeing if the seller (or forger) of the stamp can get away with it, but really. I believe that the fates stepped in and made the error for a reason. Silly? Maybe, but it's like you're willing to put out the big bucks for an original painting and knowingly getting a fake and accepting it. Wanting to willingly own a reperferd stamp doesn't make sense to me, but then again banana and mayo sandwiches just never made sense to me either.
