I put partially filled plastic bottles of water in my freezer. Before I leave to run errands, I take one out, which now has a large chunk of ice in it, fill it with water and I have cold water for my trip.
The other day I took one out that had been in there at least overnight and noticed that the water was still liquid with no trace of ice. When I unscrewed the lid the water turned into icy slush!
Why?
i think its because of Supercooling
Search online for “supercooled water” . there are quite a few pages about it, as I recall.
WAG: For some reason the water was super-cooled. I’ve had similar things happen when cooling down my Smirnoff Ice - that is no ice in the bottle and with opening up the bottle, the change in pressure causes slush.
I’ve seen the exact same thing happen with bottles of beer that were put in the freezer to chill quickly. It also happens occasionally in the lab I work in (not with beer, though).
Supercooling it is.
I wish there was a way to duplicate this reliably. I once had a 2L bottle of Tonic Water do that. It was the best damn drink I ever had. I’d love to have all my soft drinks supercooled like that.
So you drink Smirnoff Ice in the lab? I’m confused. 
No, just various solutions n’ stuff. 1 mg/ml G418, for example.