There may be other unforeseen breakage primarily in the handbook portion of the wiki.
The Table Of Contents section of each "PC-BSD® Users Handbook" page may be the best workaround.
A much improved variation of that above 'standard' also encloses the text within a similar box. Even though the text is a short paragraph of four sentences in length, it does not have the same flaw as before. The text is neatly contained within the dashed lines of the box, and also does not require the initial space character at the start of the first sentence. If that character were included, it would display with two nested boxes.