Always remember... DFRPG is big on collaborative story-telling. The Fate system is based around dragging the players, enthusiastically or kicking and screaming, deep into the creation of the story. City creation and character creation is steeped in it. The rules are based around it. Cassandra's Tears is just a tool to that end, and like many tools it has two sides. On the one hand, it's an incredible and effective way to introduce new plot hooks and arcs, suspense, tension, and role-playing opportunities. On the other hand, it has the risk of being turned into a Plot Hammer.
I'm long-winded, but bear with me. I hope this story will illustrate my feelings.
My historically favorite RPG is Amber Diceless (which, if I remember right, Fred called out as a big influence on Fate). I've played a bunch of small little mini-campaigns, single arcs, and one-off sessions, but I had one really good campaign that lasted a long time. It involved five characters; a prince and a princess of Amber, a prince and a princess of Chaos, and one guy who came from the Shadow-lands with no clear origin. The entire campaign revolved around this one Prophet, and a Prophecy relating to the tree Yggdrasil and his effect on an upcoming conflict. It walked us through the prelude to the war, through the entire war itself, and we ended it when the dust had mostly settled. The GM was a local college kid (the rest of us were in High School - the GM was one of the players' older brother), and he planned out most of this storyline beforehand. He's a fiction writer, and he turned it into this big thing. The plot he wrote was long and involved, included the fall and re-creation of Amber, a near apocalypse in the Shadowlands. Really good end-of-the-world type stuff.
The game we ran was nothing like that, because of this one damned clause he threw into the prophecy.
Don't get me wrong, the game was incredible. It was just *nothing* like he'd intended it to be. He ended up scrapping almost his entire story, and running something completely new. See, he'd included one of those really really ominous clauses about "...And one of the people in this room will die a senseless death!" or some such. Maybe it was dieing alone, or maybe it was dieing with hopelessness in their heart. But two of the characters were siblings, and another pair were lovers, and the fifth was pretty well set on immortality... So what had been intended to be a minor plot point and/or motivational factor ended up being the center of every single character's focus.
The counter-point to this is that he *could* have then used another aspect of the prophecy to hammer us back into line, but didn't. He let us run with it, and we eventually did save the universe... But it was more as a side effect of saving ourselves.
Cassandra's Tears, and prophecy in general, are awesome. They're wonderful tools. But they're *frigging dangerous*. Never forget that.