As you probably know, I’m using SyntaxHighlighter Evolved to post syntax-highlighted code. Some people mailed me, that in some of my posted code the underscores are not visible (although copy&paste works). Well, the culprit is the line-height attribute of the SyntaxHighlighter Evolved cascading stylesheet:

.syntaxhighlighter,
.syntaxhighlighter div,
.syntaxhighlighter code,
.syntaxhighlighter table,
.syntaxhighlighter table td,
.syntaxhighlighter table tr,
.syntaxhighlighter table tbody
{
	margin: 0 !important;
	padding: 0 !important;
	border: 0 !important;
	outline: 0 !important;
	background: none !important;
	text-align: left !important;
	float: none !important;
	vertical-align: baseline !important;
	position: static !important;
	left: auto !important;
	top: auto !important;
	right: auto !important;
	bottom: auto !important;
	height: auto !important;
	width: auto !important;
	line-height: 1.21 !important;
	font-family: "Consolas", "Bitstream Vera Sans Mono", "Courier New", Courier, monospace !important;
	font-weight: normal !important;
	font-style: normal !important;
	font-size: 1em !important;
	/*min-height: inherit !important; *//* For IE8, FF & WebKit */
	/*min-height: auto !important;*/ /* For IE7 */
	direction: ltr !important;
}

Simply increasing the line-height from the default 1.1em to 1.21em in wp-content/plugins/syntaxhighlighter/syntaxhighlighter/styles/shCore.css did the trick.