This is caused by fonts having a wrong UNDERLINE_POSITION property.
Examples are the font 7x13 on XFree prior to version 4.1, or the jmk
-neep font from the Debian xfonts-jmk package. To circumvent this
-problem, set x-use-underline-position-properties to nil in your
-`.emacs'.
+neep font from the Debian xfonts-jmk package prior to version 3.0.17.
+To circumvent this problem, set x-use-underline-position-properties
+to nil in your `.emacs'.
To see what is the value of UNDERLINE_POSITION defined by the font,
type `xlsfonts -lll FONT' and look at the font's UNDERLINE_POSITION