Lyle points out what I've been thinking, while reading this thread - it's going to depend very much on what you believe actually happened - that is, was it (as Emilien suggested) a single block of actions? If so, I'd back up, just as Emilien said (with certain exceptions, see below). Or, if - as Lyle mentioned - there's a pause for reaction from the opponent, it's no longer a single block of actions, and some of what just happened might not get rewound.
Remember
Chromatic Sphere? That card led us to a policy change, regarding the rewind of illegal actions. The philosophy behind that was not only the knowledge gained, but a card changing zones. If a player were to use a Sphere (or its cousin, the Star) as part of a single block of actions leading to the illegally-cast Bring to Light, then we aren't going to rewind the Chromatic portion of that block of actions. The mana generated Chromatically is still in their pool, but we can untap lands and put Bring to Light back in their hand (and issue the GRV).
**Be sure to investigate, to make sure they aren't purposely committing that GRV - perhaps because the card they drew from Chromatic Star/Sphere gave them reason to rewind**
To me, a player who cracks a fetch land as part of the single block of actions should be held to that - because it moved a card from their library to the battlefield, and re-ordered the library. Not rewinding that fetch land isn't going to hurt them in most (non-corner) cases, and, even though we have perfect knowledge on what they fetched, and can put that land back in the library and the fetch back in play, I would not give them the chance to fetch for a different land, just because they goofed while casting a spell.
d:^D