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...)