So… At the grocery store this morning I bought a gallon jug of water. Said water is a brand name which implies the water is absolutely pure (Not naming the specific brand because I haven’t read the rules on this board regarding naming brands and I don;t feel like looking it up at the moment)… The label, in fact, says “Purified Drinking Water”. The nutrition panel lists no additional ingredients in the package. Just… Water… Why, then, is the bottle plainly labelled “Best By 09/05/04” ? Does water spoil?
Oops… Didn’t mean to double post… Can someone please delete one or the other of these?
As far as I know, the seals on the bottle aren’t 100%, so in time air can leak in and make it taste funny, just like if you left it open.
It won’t really spoil but the plastic taste will leech into it. Old, plasticy tasting water is pretty kinda icky…
You ever tried a old can of water? Yuck!
Be glad we have plastic nowadays. 
Plastic water bottles are made with phthalates, which mimic female hormones and have been shown to even go as far as changing the gender of fish.
The longer the water sits in the bottle the more phthalates leech into it. Kind of makes it an estrogen cocktail.
So, no, not glad we have plastic nowadays. I’d prefer glass, which is much more recyclable.
Who would’ve thought that water could turn us into women? 
Well, the scriptwriter for one particular episode of Northern Exposure, for one. (That particular water didn’t do the womenfolk any favors, either.)