In the proofreading interface of Wikisource, with "easy LST" enabled, sections can be created in the middle of a paragraph, without creating a paragraph break:
Some text ## section ## without a paragraph break
However, these do not survive a second edition. After saving the page, editing back turns the code into:
Some text ## section ## without a paragraph break
which, when saved again, results in an undesired paragraph break.
Discussion: https://en.wikisource.org/wiki/Wikisource:Scriptorium/Help#Starting.2Fending_sections_mid-paragraph