It turns into a serious problem if the floats are of different magnitude. Consider [1e50, -1e50, 1e3, 1e3]. Evaluating it as (((1e50 + -1e50) + 1e3) + 1e3) gives 2e3, evaluating it as ((1e50 + 1e3) + (-1e50 + 1e3)) gives 0.
You get similar issues if you're trying to add a large number of small-magnitude numbers to a single larger-magnitude number. (((1e3 + 1e3) + 1e3) ... + 1e50) is substantially different from (((1e50 + 1e3) + 1e3) ... + 1e3).
You get similar issues if you're trying to add a large number of small-magnitude numbers to a single larger-magnitude number. (((1e3 + 1e3) + 1e3) ... + 1e50) is substantially different from (((1e50 + 1e3) + 1e3) ... + 1e3).