I'm not really sure where to post this, but this may have implications for our discussions of spoilers this season, so I posted it here.I have found evidence that Jeff Probst can make judgment calls concerning reward and immunity challenges.
In the first episode of the first season, while explaining the rules to the Quest for Fire challenge Jeff said the following:
"Don't put me in the position of having to make a judgement call. I don't want to, there's too much at stake."
This is a direct quote. I reviewed the tape three times to ensure I got it correct, word for word.
He isn't shown making this statement again, either in the first season or in the second season.
During the S2 ep10 ic (fire building) Probst gave Keith a warning about cheating. From Keith's reaction it seemed he understood that Probst has authority during challenges, this is probably explained to all the players before filming begins."If the race of man should be left naked upon a desert island, we should become extinct in six weeks. A few individuals might linger, but in a year would become worse than monkeys." (Samuel Butler, "Erewhon")