*** POTENTIAL SPOILERS ALERT ***
Time travel as a conceit in writing and plot development is not new. However, the writers often take liberties in how they implement the time travel and very few try to make causality work in any definite way. Often it is left up to the reader or viewer to "make up" a way where causal relationships still hold true. Heroes started off very promising with a time travel character, Hiro, who seems to be bound by strict rules of causality and what I will refer to as Linear Time Travel. Last night's episode - where Hiro travels to a dark future and witnesses...
While reading A Song of Ice and Fire, I kept coming back to the question of who is Jon Snow's mother. If you haven't read the first four books (the only ones published at the time I am writing this), then you probably want to stop reading because I'll mention stuff that you probably don't want to know until George R. R. Martin reveals them to you - it's so much better that way.
So, I tinkered with a variety of ideas and then a crazy one came to me - what if Eddard isn't Jon's father? A lot of stuff starts making sense then: The urgency of which Eddard has to tell Jon... something....