Ben 10: Alien Force, Season 3

Professor Paradox:
Has been quite a day, hasn't it?

Gwen:
[startled] Would you stop doing that!

Professor Paradox:
Oh. Very well, of I go. [walks away]

Gwen:
No! Please, don't go anywhere!

Professor Paradox:
Actually in my case, it's less of a "where" than it is a "when".

Gwen:
You know what happened here?

Professor Paradox:
You happened. When you went back in time and kept Kevin from mutating.

Gwen:
But that was the plan.

Professor Paradox:
And every plan has unintended consequenses. For instance, Hex and Charmcaster tried to take over the world, as is their want. You, Ben and Kevin swung into action, you were humanity's last line of defense. Only, there was a weak link.

Gwen:
Me?

Professor Paradox:
Oh no, dear. Not you. Kevin. Because you went into the past and undid that explosion, you kept him from mutating. Ergo, the new Kevin, or, eh, the old Kevin, lacked the powers that would have allowed him to stop Hex and so the bad guys won.

Gwen:
I didn't know.

Professor Paradox:
I warned you that time travel is for immortals and fools. I'm the only immortal hereabouts. You do the math.