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One of the examples inside Google was similar, I think it was the ordering of the keys in a Python map (that has no documented behavior). Some upgrade changed i

• tialaramex 5y ago
  It's possible you're thinking of Swiss Tables, Google's unordered hash map (and hash set) implementation for C++. There's a CPPCon presentation where the conceit is Hyrum interrupts the presenter each time some minor improvement to the design trips real users, reflecting Google's real experience doing this. But Python's dict type had such terrible performance characteristics that they actually made it much faster [edited to add: and smaller!] while also adding guaranteed ordering in current 3.x versions, which is pretty extraordinary for such an important built-in data structure.