If you have Rattlechains in play, and then you cast Talent of the Telepath and choose another Rattlechains as your selected card, then chosen card is only getting 6 mana instead of the 12 that it should (6 from Talent of the Telepath and 6 from the Rattlechains in play).
Also related to Rattlechains as mentioned in another post, it’s only giving 6 mana to the first spirit in hand instead of all of them as stated on the card.