Temperature, It seems like a simple concept gets very complicated at very low temperatures.

For example as the gas molecules are no longer colliding quickly at low temperatures, it's possible to get systems of gases where they have one rotational temperature a different vibrational temperature and a third translational temperature.

Molecular beams you can even have multiple translational temperatures. One temperature in the direction the beam is traveling and a separate temperature going perpendicular to the beam.

Essentially temperature as a simple concept just breaks down and stops working as you get close to zero.

So you have different definitions of temperature that are mathematically based.

Some excited state physics ends up having technically a "negative temperature" when you apply those math definitions. These various excited states systems are impossible to reach by heating up the material. (Like the active part of a laser). You apply the same math used to systems close to zero to these other objects, they spit out negative numbers. But it's really not temperature in those systems anymore, at least not in the way that a normal person talks about temperature.