It depends on how you define "improve". I wouldn't call a runaway AI an improvement - from the users' perspective. E.g. if you think about the Chernobyl power

It depends on how you define "improve". I wouldn't call a runaway AI an improvement - from the users' perspective. E.g. if you think about the Chernobyl power plant accident, when they tried to stop the reactor by lowering the moderator rods, due to their design, it would transiently increase the power generated by the core. And this, in that case proved fatal, as it overheated and the moderator rods got stuck in a position where they continued to improve the efficiency of the core. And you could say that it improved the efficiency of the system (it definitely improved the power output of the core) but as it was an unintended change, it really lead to a fatal degradation. And this is far from being the only example of a runaway process in the history of engineering.