Reanimation of Yondaime during Chunin exams I read everywhere that Orochimaru tried to reanimate the 4th Hokage during the Chunin exams, but he was unable to reanimate him because he was sealed inside Soul Reaper's belly. But if he can't reanimate him, what was the point of the coffin? To perform reanimation, he needs a host first. He must have reanimated him earlier on a host if it was possible. So what was the point of summoning that coffin?

According to the Naruto wikia's page on Summoning: Impure World Reincarnation (specific revision),


  To perform this technique, the user must first acquire some of the DNA of the person they intend to reincarnate. Kabuto remarks that this basically amounts to grave-robbing, although blood stains or organs salvaged after the target's death also work. The soul of the intended reincarnated must also reside in the pure world (浄土, jōdo); for example, those whose souls have been consumed by the Death God cannot be reincarnated. However, if it is a case where the souls are freed from inside the Shinigami, then a user of this technique is free to reincarnate them once again.
  
  Next, a living sacrifice is required for the soul of the reincarnated to use as a vessel.
Remember the Orochimaru did not know of the Shiki Fuujin, he didn't know about not being able to summon the fourth. The Shiki Fuujin was kept highly confidential and was only ever used twice throughout the series.

Now for a more practical explanation, the author likely didn't think that far ahead, and caused The Third to be able to "cancel" the third summon. The Shiki Fuujin explanation was later applied an implied by certain events taking place.