On page 196 of the deathly hallows, Kreacher the house elf explains how he went with Regulus Black to the lake where Voldemort hid the locket. He gave Kreacher a fake locket, and told him to replace it with the real one when the basin was empty. He then drank the potion. Earlier, Voldemort made Kreacher drink the potion, and he then REFILLED the basin with potion after he dropped the locket. Later on, if Regulus went back to drink the potion and replace the locket, how was there potion in the basin when Harry and Dumbledore went there to get the locket in the previous book?