I had been pasting Google adsense code into a Static sidebar, such as:

<script type="text/javascript"><!--
google_ad_client = "pub-6429360491891980";
google_ad_width = 120;
google_ad_height = 600;
google_ad_format = "120x600_as";
google_ad_type = "text_image";
google_ad_channel ="";
google_color_border = "CCCCCC";
google_color_bg = "999999";
google_color_link = "000066";
google_color_text = "333333";
google_color_url = "FFFFFF";
//--></script>
<script type="text/javascript" 
  src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</s>

But when I would return to edit my sidebars again, I would get

<script type="text/javascript"></script>
<script type="text/javascript" 
  src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>

because my browser (Safari) would not populate the textarea with the commented code. Thus, I am escaping the HTML for the textarea.