Anecdote follows.
A while ago, I had stocked up on bottled water for something, and had a case sitting around for a while. On my way to a class one day, I grabbed a bottle to take with me. Later that morning, I took a sip. As I was setting the bottle back down on the table, I spotted "something" floating at the bottom inside the bottle. It was a dark gray-ish filmy thing. I then noticed the bottle was way past the expiration date. Needless to say, I didn't finish the bottle.
I went home and looked at the remaining bottles...several had the same filmy thing inside so I chucked the entire lot.
I emailed the bottling company. They said this was common for 'expired' water and sent me some coupons.
These days, I don't keep an inventory of bottled water for any extended length of time.
omni