Investing everything in engines and abandoning battery development in the early 1900s. Lead-acid batteries were heavy but usable, and electric cars were more popular until electric starters were added to engines. A disproportionately big, short-lived reason was the lack of sufficient electrical grid for electric cars trying to go far.
Nobody in government was thinking ahead, so everyone was forced to trying to make their own money NOW, and that's how we get inhumane tech in general. Same thing happening in computers for decades now. We need centralised R&D free from market influence for the benefit of all life.