A few points on vodka:
1) If ever there was a place for analytical chemistry in the drinks business,
this is it! I'll bet that there's more in there than just ethanol + water.
2) Congeners are human-detectable at really low concentrations, so it doesn't
take much to alter the taste/smell.
3) Just because the law says it must be water + ethanol doesn't mean that it's
true in practice. When dealing with pure things, cleanliness is everything.
Perhaps a certain vodka was distlled to 190 proof, but later handling
could "inadvertantly" add flavorings. <*see footnote>.
4) I'm not joking when I say this, but: define "water". As we've seen in
the press lately, Coca-Cola's flagship bottled water ("Dasani"), marketed
as "pure", actually has added magnesium, calcium, and sodium bicarbonate,
all for taste. How much of these minerals can you add and still call it
water? How much of these minerals are required to make vodkas taste
different? I consider minerals to be valid ingredients in vodka which
might very well lead to detectable differences in taste.
<*> I heard a funny story about Budweiser trying to track down some "off"
flavors in one of it's beers. They thought they had an infection
somewhere in the brewing process, but it only turned up every now and then.
They pulled their hair out for a while... and discovered that it was coming
from the bottle caps. So they leaned on their suppliers and tried to root
out the problem, but to no avail. Eventually they discovered the real problem:
every now and then, one of the semi trailers used to haul the bottle caps
would have a residual smell in it from a previous (unrelated to beer) load
of something they were hauling. This would then seep into the plastic lining
of the bottlecaps and from there seep into the beer!