On the E190, the joints in the ductwork are less than brilliant, which can cause sensors around the cabin to read the temperature incorrectly so sometimes even though the temperature is set to 21° (roughly… it’s controlled by a rather vague dial) you can have one half of the cabin roasting hot and the other freezing cold - you can actually feel the temperature change dramatically from one row to another!

The system also has a tendency to run warm when switching from APU to engine, then overcompensate during the climb by blasting cold air out of the vents, and then fight itself for a bit before settling down.