I got in my car this morning and the temperature read 13 F (that's around -10.5 C.) While driving down the road, I noticed a bottle of water in my cup holder was frozen solid as expected.
Another bottle of water (cheap, thin-walled, plastic disposable water bottle) contained liquid water! I shook the bottle, not a single crystal of ice!
Shortly thereafter, I noticed the water became slushy, so it had begun freezing quite quickly!
The interior of the bottle was hardly perfectly smooth given the quality of the bottle and the water was half drunk so it was probably not super pure.
What caused this? Was this supercooling?