I can certainly tell Coke and Pepsi apart in blind tasting. I've more than once requested Coke, had it served in a Coke glass, thought "this tastes a bit odd" and checked at the bar to find out that it was coming straight out of a Pepsi dispenser.
so it's not that the undergrads are particularly dumb - it's that expectation is often more powerful than experience unless you consciously control for it. for example, i suspect it's easier to detect the difference between coke + pepsi when you're given an unbranded glass than when you are given the two in branded bottles, but swapped (and, importantly, not expecting it).
The truth is that real experts can distinguish these differences - many trainees many not be able to, but that that's a different matter.
And as I've mentioned, I've been able to tell the difference between Coke and Pepsi even when all environmental clues (my order, my expectation, the type of glass) have told me that it's the other one. Again, it's highly likely that these environmental clues often influence people a lot more than they realise, but that doesn't equate to the claim that experts can't see through these either.
I agree with you 100% on the Coke versus Pepsi thing. Anyone who has spent much time drinking wine has wines they like better than other wines; there really are taste differences. (Note that I'm not saying more expensive is better; rather, that there are $15 wines I love, and $15 wines I don't like.)
In your experiments, you know, at least subconsciously, that the possibility exists that you will be served Pepsi. In proper experiments the subjects don't know what they are tasting. You merely ask them for a preference. You also make it clear that both glasses may contain the same liquid (and preferably the experiment leader doesn't know which is which). In such cases, the result holds: M is preferred.
Secondly, it's a statistical result. It doesn't mean there aren't people that can't taste the difference.
As for the wine:
Many experiments are flawed. I don't know what experiments you saw, but if you ask experts to distinguish between a glass of red and a glass of white wine, of course they will succeed. The expectations have been set properly. One white, one red. In all cases where people are asked to distinguish between two different things and they are also presented two different things, they will succeed.
That does not mean they won't fail to note something is white wine when you give them a glass of seemingly red wine and ask them to rate this red wine.
The point is not that there isn't any difference between wines. There is and oenologists will be able to tell you the difference... as long as you told them upfront that there is a difference (and there is indeed that difference).
As for the the statement about my Coke v Pepsi - first time it happened, I had as much idea that it might be Pepsi as the people in this experiment had that they might have been served white wine masquerading as red. I wasn't particularly thinking about it. I'd ordered a Coke, the drink came in a Coke glass and I was fully expecting to drink Coke. My first thought wasn't "Oh, this is Pepsi". It was "There's something funny about this Coke".