We don't plan to intercept all attempts to mutate a list while it's being iterated.

For iterating with an Iterator, it's not possible, because we don't know when iteration is done.
For forEach, it would be possible, but only at the cost of an overhead on every mutating access, even when you are not iterating - something like:
  add(x) {
    if (isIterating) throw error;
    ... actual adding ...
  }

We try to optimize for correct programs, not errors, so it's ok that the erroneous program does something bad before it's clobbered. We throw an error, and it's perfectly reasonable to terminate the program as a result.

So, this boils down to a request for better documentation.

Notice that generally we write "It is an error if ...". That does not mean that doing so will always throw an Error. It likely will, and we try to do so, but it rally just says that "You must not do ...", and if you do, the result is unspecified.

Removed Type-Defect label.
Added Type-Enhancement, Area-Documentation, Triaged labels.

We should make the API docs clearer (this problem is in lots of places) and also provide a doc comment guideline to encourage better wording.

Added Docs-API, Docs-Articles labels.

Removed Type-Enhancement, Area-Documentation labels.
Added Type-Documentation, Area-Library, Library-Collection labels.

It looks like the documentation here still isn't terribly great. @kwalrath is there something better we can change this to?