Not a single underscore appears in the reply box, however, as you can see, several underscores appear in the quoted text. (To see the text as it actually appears in this reply box, click the small “reply” link at the bottom of this post.) It seems that every instance of the word “about” (completely case-insensitive) followed by a colon is displayed as a completely lowercase “about” followed by an underscore followed by a colon. It doesn’t seem to happen for words other than “about,” and the final colon appears to be necessary for the bug to show up.
It’s because “about” followed by a colon can be used to form pseudo-URLs used by some browsers. In Netscape I know that if you just type in a-b-o-u-t-: it brings you to the same screen you get if you pick Help, About Netscape. The word “javascript” followed by a colon can also make these pseudo-URLs to execute JavaScript code, so j-a-v-a-s-c-r-i-p-t-: becomes: