> By definition a liquid cannot be below its freezing point until it has completed its phase change.

Supercooled water is a thing: (and I've seen it a few times myself)

https://en.wikipedia.org/wiki/Supercooling

Superheating, too, and that's something that can happen in a microwave, occasionally causing severe burns when superheated water flash boils. AFAIU, it's the same phenomenon--lack of nucleation sites to kick-off the phase transition.

Does that mean every TTT diagram is wrong?

Nevermind I just learned about CCT diagrams. I guess having only one year of materials engineering means I should STFU. :)