Mutation isn't necessary to demonstrate the problem. Consider three libraries: first, there's a basic, widely-used datetime library. There's also a timezone library depending on datetime 1.0 and a dateformat library depending on datetime 2.0.

  # Get the current time in the PST time zone (returns a 1.0 object)
  now = timezone.now_in_zone("PST")
  # Format the date for display (accepts a 1.0 object but depends on 2.0)
  formatted = dateformat.format(now)

Now the problem: imagine that datetime 2.0 switched from one-indexed months (1 is January) to zero-indexed months (1 is February). The timezone library depends on datetime 1.0, so it used "1" to indicate January, giving me a datetime value with month=1. The dateformat library depends on datetime 2.0, so it incorrectly interprets that month=1 as February. All of my January dates will now be incorrectly formatted as February.

(Switching month representation is a rather drastic example, but it's also clear. Substitute a more subtle data format change if you'd like.)

There's no mutation here, and the dependency graph is trivial. I just received a datetime from one library and passed it to another library.

It's possible that I'm missing something, but I've been asking Node users about this for a couple years and I usually get blank stares. I also have no horse in this particular race. Vendoring seems like a great idea to me, but I fear the uncertainty of this version mismatch situation.