h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Tonight I had a large bottle of carbonated water (rather than naturally fizzy) which I wanted cold so stuck it in the freezer for an hour. I do this frequently, sometimes getting slushy water but normally just get very cold fizzy water. I poured a pint glass of the water which was cold but not frozen and had no ice crystals so as it was hot outside I chucked in a couple of ice cubes. I do it most evenings and get the usual small fizz of ice going into a carbonated drink.

As I took it outside to a warm external temp, the whole glass formed a huge iceberg of fairly solid, thick slush with my icecubes perched on top, bulging over the top of the glass. Like a Coke float but much more dense slush that I couldn't poke my finger through.

Can anyone explain why tonight's water was so different from any other nights? Extra carbonation? Difference in temperatures?

The gas dissolved in the water will usually keep it from freezing. Then once the gas is released, the cold water is able to turn to ice. Perhaps in this case, you had left the bottle in the freezer for longer, causing the water to get to a lower temperature than normal.