Mistakes on DVD covers?

It depends. The only way they become valuable is when they are also rare and desirable. There are many out there, like Saving Private Ryan, that are too pervasive to have any additional value. And sometimes, you may think it's a mistake, as with your Saving Private Ryan, in actuality it was intentional. Ofcourse, spelling and grammer errors are true blunders, but again, produced in too high a quantity to have any additional value.