I've been fighting with the mini-split heat pump in the main living area of my house. Previous owner had it put in, installed by fuck knows who - no "call for support" sticker on the unit or anything. Anyway in the summer it'd cool the house fine, but in the winter you'd set the thing to 18 degrees and it'd heat the room to 23 degrees.
The thing's got secret menus and settings in it and you can program in a temperature offset, I figured it was shit like that which was causing the temperature to be wrong, and was about to call up a local LG installer to find out wtf was going on. Finally posted on hvac-talk asking for suggestions, someone asks... "is the hole in the wall behind the unit insulated?"
Dug into it... NOPE. Wide open hole through the house. The thing was pulling in cold air through the hole, which was cooling down the temperature sensor in the thing and making it heat more to compensate. Filled the hole with spray foam and now the room is sitting at its setpoint.