"3idiots" is one of the great movies acted by Amir Khan. It's actually Raju Hirani’s movie. A great movie maker, I am a big fan of both of them. And we all know that When Amir Khan work in any movie, he tries to make that total movie perfect, and mostly he succeed. So, to find a mistake from their film is very rare.
In "3idiots" when Raju and Rastogir went to Rayanshu's home to find him they saw that Rayanshu is totally a different person. to make him say about their friend Rayangshu they took a pot of his father's ashes and blackmail him that if he told tell the truth they will drop it in the commode. To make it look real the drop the head of the pot. New Rayanshu get scarred and tell them everything. So, Raju returned the pot. And on that moment we see that pot is again replaced (which is already dropped in the commode).