Revealing mistake: When Mrs. Petrelli is sitting with the dead Peter, just before she lies down on his chest and starts to cry, you can see the "dead" man tilt his head slightly backwards. (00:21:10)
Continuity mistake: When Sylar comes for Isaac the painting of Hiro and the dinosaur is visible in the studio in several shots, even though Hiro took this painting to Linderman several episodes ago. The episode director even comments on it during the audio commentaries.
Chosen answer: Because he's an idiot. From the story point of view, they needed the vault to actually be physically open so that the release of the virus would be a credible threat. Unfortunately, this required that Peter conveniently forget that he could simply walk through The Wall and use his telekinesis to tear the door open instead. Peter's increasingly large arsenal of abilities, where he ended up having pretty much any ability required for any situation, led to a situation where it became necessary to either have Peter apparently forget that he had a given power, as with the vault door scene, or become naively trusting, as with his refusal to scan Adam's mind to confirm his good intent. This problem, which simply made Peter look increasingly dumb as the show progressed, may well have been a key factor in the decision to severely reduce his ability during season three.
Tailkinker ★