The way I see it, the mirror does flip up and down, at least by the measure of the same experiment used to prove that it flips left and right.
To prove that it flips left and right, imagine a vertical pole between you and the mirror. The pole has a card attached to it with the front facing the mirror. The two dots, red and blue are painted on the card and you can see their reflection in the mirror.
In the reflection, the blue dot is on the right and the red dot is on the left.
If you rotate the pole so that is facing you and you can now see the front of the card you see that the blue dot is really on the left and the red dot on the right.
This proves that the mirror flips left and right.
To prove that it flips up and down, imagine the same pole in front of you with the card facing the mirror, but this time the pole is horizontal.
in the reflection you see that the blue dot is at the top and the red dot is at the bottom.
If you rotate the horizontal pole so that the card is now facing you, you see that the blue dot is really at the bottom and the red dot at the top.
In practical terms though, all they're storing is a key. The actual data is held elsewhere. In the same way, an entity tag on a cached object is like a key to identify whether the object has been modified on the server since the last time it was sent.
How would it be possible to spot that it was being used for tracking a user rather than just part of the normal functioning of the browser?
I see this also covers the use of Flash cookies, but I wonder about the use of Etags as a tracking mechanism.
If I recall correctly, some of these sites use cookies, Flash cookies and also unique Etags on an object in the browser cache to try to work around people blocking cookies from their domains.
This is a long-standing argument of mine. Some of our users maintain that they are only interested in the functionality rather than the quality of the code/design/integration etc.
Then, six months later, when they need an enhancement and find that it is going to take twice as long/cost twice as much to build something on top of the quick & hacky solution that they originally insisted on, they still make unhappy noises.