You may guess that this item appeared in FAQ after numerous
discussions on this mailing list. Search its archive for details.
The most close feature is inline tasks, there are other structures
like #+begin_something/#+end_something blocks and :drawer:...:end:
that have closing marker.
Some Org syntax limitations are imposed to be able efficiently
determine current context using regexp searches. So "** Heading" lines
must be escaped in comments, nested identical #+begin_block structures
are not allowed. Another point is treating Org files with syntax
errors.