[Spivak]

Yep! Dumb example.

    main()
      x = get_from_some_external_data_source()
      if x:
        print("Hello World")
        trigger_ub()

You might expect this code to always print if x is true but the optimizer can look at this and say "welp, if x is true then it would trigger ub, therefore it must be false, and since x must always be false we can just remove that entire branch."

[kbenson]

My favorite example along these lines (in C) is "Cap'n'Proto remote vuln: pointer overflow check optimized away by compiler"[1] which was covered here a few years back and shows all of these "theoretical" compiler behaviors coming to a head in a real bug which is thoroughly explained.

1: https://news.ycombinator.com/item?id=14163111