If text in Panel's pop-up contains the "_" character, the character disappears when pointer leaves the window
usually the character _ disappears completely (joining more words into one), other times it turns into an underline of the following character. this happens also sometimes when the pointer leaves the window without losing the focus (no subwindow opens), the most likely in the "Bookmarks" menu on the Desktop or in Nautilus, if you have a bookmarked folder containing this character in the name.
this is obviously caused by the fact that the underlines of a letter mark the keyboard shortcuts to activate a function without clicking on it,, but when the window loses the focus, this character disappears even if it is a regular part of the text...
watch the screencast
DistroRelease: Ubuntu 11.10
Package: unity 4.24.0-0ubuntu2b1
Uname: Linux 3.0.0-14-generic i686
Date: Thu Jan 5 20:05:53 2012
InstallationMedia: Ubuntu 11.10 "Oneiric Ocelot" - Beta i386 (20110921.2)
PATH=(custom, no user)
UpgradeStatus: No upgrade log present (probably fresh install)