Some of the tiles above volcanoes get filled with very high mass very hot igneous rock upon loading the game. It was not there when the game was saved.
Steps to Reproduce
Build structures above a volcano to trap and convert the heat to power. After some time of successful operation, when the game is reloaded this very hot igneous rock appears (randomly?) above the volcano and overheat everything.
This issue has not been confirmed by a developer yet.