As I interpret it, everything that El Nino says is technically correct, but I agree with Dan-Cat in how he handled the re-spotting of yellow onto its proper spot. I am firmly planted on the fence and in the middle of the road and waffling on this subject.
Section 5. of the Rules covers the definition of the Officials and their duties and responsibilities. However, this really only deals with the game proper when real living, breathing "officials" are tending to these duties. It doesn't cover simple, social games wherein one's opponent is ACTING as the referee. Specifically, Section 5., Rule (1) (a) (i) and (ii) essentially grant the Referee supreme power to make a decision in the interest of what the referee considers to be fair play.
In a tournament match, I once had a situation nearly identical to Dan-Cat's. No referee, opponent was responsible for the spot. So I am working the Black end with a couple of reds and the Green is down there as well and the last Red is in the jaws of the Yellow pocket, Yellow and Green Spots are open. So I have the table worked out in my plan and proceed to take a Red, pot Green, and I am focused on the next Red-Black sequence. The opponent/referee calls out "Four!" and I rapidly pot my Red with a nice angle on Black, pot Black, and run White down to baulk to take care of the final Red. White stops perfectly where it already had in my imagination about 3 or 4 inches on the black side of Yellow Spot in perfect position for the hanging Red to leave on the next nice colour. Except for the big, fat Green ball sitting on the Yellow Spot, a perfect snooker. F*@%!
Yes, technically, by the Rules, I am supposed to be responsible not only for working out my break, performing the mental arithmetic of the scoring, and the required mechanics needed for execution, but I am also supposed to be responsible for my idiot opponent not even being competent enough to know which spot to put a ball on. My fault. My opponent was actually a nice guy, just frustrating to play against for his repeated showing of ignorance of the actual game. So I consulted the Tournament Director, a bona fide professional Referee (same one from which I received my training). He requested from my opponent that the Green should be re-spotted to its proper spot as the situation was not the fault of the striker and, though it did not APPEAR to be, the placement of the Green on Yellow Spot, COULD HAVE BEEN interpreted as an attempt by the opponent to impede access to the Red in the Yellow corner. To repeat, this was a REQUEST from the Tournament Director and my opponent (again, nice guy) agreed to this. It did not come to it, but my gut feeling of the situation was that, had the opponent refused, the Tournament Director would have just referred to Section 5. and overruled the opponent anyway, but perhaps, he may not have and could have just told me that it was my problem, deal with it.
There was a situation a few years ago (strictly memory here, so I may get specific details wrong), I believe it was an event in China with, I think, Stephen Maguire against a young Chinese amateur. Like English is the international language for air travel, so it is the international language for Snooker. Anyway, the young Chinese played a nice Red and, not speaking or understanding English well, verbally called "Blue", then proceeded to play a perfect Pink which would have continued a nice sequence. "Foul" called by the Referee and rightly so. The Chinese kid was confused at first but accepted his mistake, but Maguire (or whoever) refused to accept the foul and requested the referee rescind the foul call as the striker was clearly playing at the ball he struck even if he got the word incorrect. The Referee agreed to this and it was and is the right thing to do.
So the point is, while the Rules of Snooker are strict, they need not be absolute if there is an extenuating circumstance. The Referee wields a lot of power, even to the point of overruling a clear, black and white written rule if doing so is appropriate to the situation.