The slashed o for diameter mentioned in wikipedia returns the following error when used in a post, regardless of the editor:

500 - Internal server error.
There is a problem with the resource you are looking for, and it cannot be displayed.

I could probably use the HTML editor and the specific HTML entity #x2300 for it. Let's see if this works here: ⌀

But the point is that I lost all my long post, as it afterwards returned an expired post error.  ;(  (there's probably an HTML entity for that smiley too...)