I’ve been wondering how this thread managed to end up with a smiley showing up in the thread title. When creating this (originally as a test message) every time I try to place a smiley in the title, it is placed in the message body.
if you try to create a thread in the Marketplace forum you’ll see there’s a section just below the message body input box that lets you select an icon. That icon will appear at the start of the title. You can use [preview] to see it in action and then simply not post your test message in that forum.
Apparently TPTB have that option enabled for that forum but no others I’ve noticed.
As a separate matter, those icons are signaled by a colon followed by certain text or letters. There’s no difference between typing “:)” or clicking the link for the generic happy face. Either way what gets sent to the board is the 
code and what gets displayed to readers is an <img> link.
If you do manually enter into the title box, that’s what gets rendered to readers: a colon and a left paren. Likewise if you put in stuff like “This should be [noparse]Italic[/noparse] but it won’t display that way”. It renders just as you typed it, with square brackets and all
It’s interesting, but not real surprising that even when you have your typing cursor in the title box, clicking one of those icons in the editor injects the :whatever code into the body wherever you’d last left the typing cursor within the body.
The same thing happens if you click any of the other action icons while your typing cursor is in the title field, e.g. the one to insert a url or a list. In each case they inject the BBcode into the body text. Because that’s the only field that accepts anything other than plain text.
Per the long-standing Windows UI guidelines you’ll see the body text input box and all those various buttons are enclosed in a thin impressed-looking hairline. That’s the grouping symbol that tells you which controls operate together on one another. By the standard (dating back to Win95), if it’s inside that enclosing line it can’t have effects outside the line. And vice versa.
Per this test thread, it appears that we can use emoji in Titles. At least, technically: I don’t know if the mods have a rule against it.
I don’t think we have a formal rule against it, but those emojis don’t render properly on all browsers, and if it totally hoses someone’s page as a result, then we’ll usually go in and edit out the offending characters.
It’s best not to use them.
On a related note, I was wondering if maybe one of our fine members has something else to sell in the Marketplace, so that that super-classy thread title gets knocked off the main page? A small favor :).