Rift

In Collision, Future Hiro says he risks creating a rift by travelling back in time to speak to Peter Petrelli. Just what constitutes a "rift" and how Hiro knew about the risk of causing one remains a mystery.

Speculation

Hiro may have experienced a rift in Six Months Ago, when he tried to go back in time to prevent Charlie's murder. In the episode, they had just confessed their love for each other and were about to kiss when Hiro abruptly and unexpectedly jumped through time to the present. He later told Ando that he tried repeatedly to go back to save Charlie, but couldn't.

In theory, the space-time spontaneously corrected itself to keep him from creating a causality paradox; he could not do anything in the past that would keep him from having a reason to go to the past.