1: When the very flow of time colapses upon in itself due to a temporal incursion or "Time Travel" Noteworthy people who have gone backward in time would be...Well, that guy in Timecop....Shut up.
Anyhoo, don't go back in time, because you'll only crush a mosquito and destroy Berlin.
For more information on time paradoxes, Play TimeSplitters: Future Perfect. or watch a ST: Voyager episodes when it goes back in time...They have it down.
2. Whatever went on with Red Foreman and that big ship in Star Trek: Voyager. Like, seriously, WTF? Temporal Shielding? And why could they live forever if they're protected from time. They'd age normally! Sorry, it's just the fact you could erase places from time looks cool, but is just not feasible.
"What the hell, you're me!"
"Yeah, I knew you'd say that. Take this key, it's for that door. You won't make it through without it."
"If you couldn't make it through, who gave it to you?"
"Just shut up, I gotta go and save us from a big snake"
"If you're me, then-"
"I'm you right?"
"Oh yeah....How many fingers am I holding up?"
"I'm you, I'm not psychic....4"
"Really, now go on ahead through that door, and watch out for...Never mind"
You fucking up history.
It'd be like going back in time and shooting yourself in the head, you'd die, but you wouldn't appear back in time to kill yourself, so you'd have to wait until after you said you'd do it to successfully kill yourself, right? Wrong, you still wouldn't have done it, so history resets, and if you are one retarded motherfucker, you will try it again until you stop buttfucking your grandmother in her prime back in the 1940's
Adrian Shephard in Gearbox's 'Opposing Force', If you jumped into the Portal after Gordon, it would say subject terminated stating that the reason being was that you attempted to create a temporal paradox