In episode XII, Jack joins up with some mobsters who plan to steal the Neptune Jewel.

Through out the whole episode, until the very end, the Neptune Jewel is shown as a crystal tear-drop shaped.

But when the end of the show does come around, the shape of the Neptune Jewel is switched accidentally for a four-sided diamond shape.

It REALLY wouldn't have been that hard to look over the finished episode and see the mess-up but I guess we'll never know why the studio who made Samurai Jack decided not to. Ah well.