FSN13

Illya casts Power Word: Oniichan and we're at the first truly crucial moment of Fate Stay Night.
Why do I say crucial?
Because, it's at this point, in the Fate (Saber)'s path of Fate Stay Night, that two gigantic spoilers happen.
And these two gigantic spoilers are so large that, if they do happen exactly as they do in the game, they make the other paths utterly impossible.
One is archer dying against berserker.
Again, I don't think this is plausible, as you don't know jack about him yet, and for him to die without knowing more about him would just be a cop-out shame, but that's exactly what happens in Fate.
Moreover we're only on godamn episode 13!
I think there's an easy way around this, just make sure he DOESN'T DIE (escape, wimp out, but I HIGHLY doubt he'll be the one to beat Berserker, as they've already hinted at who is going to be doing the crazy shit next episode.)

Frkncrzyazn: OMG IS HE GOIN TO TRACE HER SWORD?

The OTHER spoiler is rather hard to get around.
For one thing it is rather crucial, as otherwise, Saber will disappear.
However, if they DO go that route...certain events in other paths become HIGHLY unlikely.