function rec(x, y, z) {
       if (something(z)) {
          return rec(x, y, z-1);
       }
       return 1;
    }

    function rec(x, y, z) {
       if (something(z)) {
          return rec(x, y, z-1) + 1;
       }
       return 1;
    }

To avoid this problem whenever you modify a possibly recursive function in a language with transparent TCO - you have to look through the code to see if it's using TCO. This is wasted time. And it can be non-trivial if you have recursive functions calling each other and other funny stuff.

If instead the language required special syntax for TCO:

    function rec(x, y, z) {
       if (something(z)) {
          return TAIL_CALL rec(x, y, z-1) + 1;
       }
       return 1;
    }

What is the downside to requiring special syntax?