Comment by zmonx

You can crash Emacs for example with the recipe from #2099:

   $ emacs -Q --eval "(let (v) (while t (setq v (cons v v))))"

In response to how such deep structures can arise: They may for example arise when testing Emacs with randomly generated code snippets. For testing edge cases, it is highly desirable that Emacs run robustly also in such situations.