Javascript input causes 404 or 500 error

When editing my default theme in WordPress 2.3 if I try to enter javascript into a theme file that is writable I get a 404 Error. However, I can edit that same file with any text as long as it isn’t javascript.

If I use a custom theme I downloaded doing the above task causes a 500 internal server error.

The javascript I’m trying to enter is adsense javascript cut and pasted from google’s website:

<script type="text/javascript"><!--
google_ad_client = "pub-7774992805004161";
google_ad_width = 180;
google_ad_height = 150;
google_ad_format = "180x150_as";
google_ad_type = "text";
//2007-11-07: geeksandphreaks-post
google_ad_channel = "1380497186";
google_color_border = "CAF99B";
google_color_bg = "000000";
google_color_link = "80FF00";
google_color_text = "C0C0C0";
google_color_url = "999999";
google_ui_features = "rc:6";
//-->
</script>
<script type="text/javascript"
  src="https://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>