[img_assist|nid=138|title=I�Unicode|link=node|align=left|width=460|height=83]

Stole this somewhere, I'll provide a link once I find it again.
See Unicode Specials on english Wikipedia for the "replacement character".
Update: You might see a different character beneath compared to that in the image. Forms rendered include the question mark in a black rhombus, a white block with black border or just a simple question mark.
Update2:the link