Most thermostats are “fail safe” meaning when they go bad, they are designed to stick open so you end up running too cold. But when people don’t change their coolant at scheduled times, and you start getting alpt of corrosion and scale, thermostats can jam up and fail in the safe or unsafe position causing overheating. Now there are “fail unsafe” thermostats out there, but those are typically performance high flow thermostats, and aren’t typically used on everyday drivers