In the summer our windows are usually open while it is colder outside than in. To find out when to close them I compare the temperature online to the one inside. We have an old mechanical thermometer near our back door, and an electronic thermostat in the living room. I usually check the one near the back door, but recently I have started to check both, and the thermostat was consistently ~2.5C warmer. If that difference were real, I figured that would be enough to notice on the skin as well, but it felt no different. So, I measured the temperatures of both with my IR thermometer.
Turns out the mechanical thermometer was about 2.5C out. And, it had a screw in the back to adjust it, so it shows the correct temperature now.