I can't reproduce it on demand, I've noticed a bug, I believe related to the volume of water, or surface tension changing (ie not when the water is just inert), that causes a dramatic drop in temperature (e.g. by 20 degrees or so in an instant). All the water (or polluted water) in the same pool changes at once regardless of size - I've had it happen quite a few times, in 1 tile wide sections, and also once in a 15 x 3 block of polluted water.
Changing volume of water, various situations. Sorry can't be more specific.