Seven Minutes to Midnight - S1-E8
Continuity mistake: When Mohinder is back in India, his mother tells him that his sister Shanti died at the age of 5, when Mohinder himself was 2. In "The Hard Part", Mohinder tells Molly Walker that Shanti died before he was born, and shows her a photo of Shanti (who looks 5 years old) and his pregnant mother.
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 ★