Similarly to not checking array bounds, undefined behavior was once introduced in the name of efficiency - back in the ages when the performance difference real

Similarly to not checking array bounds, undefined behavior was once introduced in the name of efficiency - back in the ages when the performance difference really mattered. And both are just a major headache now, and belong to reasons why few people start new projects in C. I wonder how many such design decisions, relevant today, but with a potential to screw up future humanity, we are making right now.