Altimeter: Hot to cold = look out below?!
It seems that based on the way the altimeter works, the pressure is directly proportional to the difference between the pressure set in the Kollsman Window, and the outside air pressure. So the greater this difference, the greater the altimeter's altitude reading.
When flying from a warmer area to a colder area, the altimeter's altitude reading would be higher than the actual altitude the plane is flying at. Since the altitude reading is based on the difference between set pressure in the Kollsman window and the air pressure outside the aircraft, this implies that the the difference read by the altimeter is too high.
So let's say that you have your Kollsman window pressure properly set to 30 Hg for a warm area - this setting would give you the correct altitude if you stayed in the warm area. Let's say that the air pressure outside the aircraft when it's in the colder area is 18 Hg. We know that air pressure decreases as altitude increases. The difference between the now incorrectly-set pressure (30 Hg) and the outside air pressure (18 Hg) is 12 Hg. But since the video said that this difference is larger than it's supposed to be, the actual Kollsman pressure setting should be less than 30 Hg to reduce the difference of 12 Hg (you can't reduce this difference by increasing the outside air pressure). So if the actual Kollsman pressure setting is 26 Hg, the pressure difference would be 26 - 18 = 8. From this, we know that the incorrect altitude reading is 150% (12 / 8 = 1.5) greater than the actual altitude.