[A mouse over will not let you see anything inside the brackets. Now that you know, use this new found knowledge wisely.] No need to click this thread. move along.
Okay I clicked on it like you said. Now what?
I did not know that. Fascinating.
Actually, this was noticed before. See this ATMB thread by bienville from last October.
It was suggested that you could use this as a trick to include spoilers in your text that will not be seen in the mouse-over preview.
It was also noted that, if you have text surrounded by BB codes, for example [noparse]Bold-face stuff here[/noparse], the text in between the start-code and end-code DOES appear, even though the actual BB codes [noparse]** and **[/noparse] do not.
Need/want.
Also see the thread “test mouseover with brackets” started by me, last post on 10-10-2012. It should be on this page at the moment: http://boards.straightdope.com/sdmb/forumdisplay.php?f=2&pp=150&sort=lastpost&order=desc&daysprune=-1&page=4
Ah yes! That thread too! Did bienville get his piece of the action (see Post #4 there)? 
Well damn, I thought I was special.
Here’s my test thread on this subject from 2008. FYI, I received no royalties. 
I had planned to do test thread for this in 2015, but I won’t be able to afford the royalties.
I fold.