We have an IHttpHandler for stylesheets and add a <link/> tag to the page like this:

HtmlGenericControl link = new HtmlGenericControl("link");
link.Attributes.Add("rel", "stylesheet");
link.Attributes.Add("href", "stylesheet.axd?d=" +
                                    HttpServerUtility.UrlTokenEncode(token));
head.Controls.Add(link);

In the stylesheet.axd handler, we UrlTokenDecode the d query string parameter like this:

var token = HttpServerUtility.UrlTokenDecode(Request.QueryString["d"]);

This works most of the time but every now and then we find one of the following two exceptions in our log:

[FormatException: Invalid character in a Base-64 string.]
   System.Convert.FromBase64CharArray(Char[] inArray, Int32 offset, Int32 length)
   System.Web.HttpServerUtility.UrlTokenDecode(String input)
   ...

[FormatException: Invalid length for a Base-64 char array.]
   System.Convert.FromBase64CharArray(Char[] inArray, Int32 offset, Int32 length)
   System.Web.HttpServerUtility.UrlTokenDecode(String input)
   System.Web.HttpServerUtilityWrapper.UrlTokenDecode(String input)
   ...

Any ideas what would cause this phenomenon?

Remarks:

• the resulting URL is < 1500, so below any known URL limits (e.g. IE: 2083)
• seems to be independent of user agent (we have these exceptions for IE6,7,8, FF & Chrome)
• our (unconfirmed) suspicions include AntiVirus products, HTTP proxies, browser addons
• found this remotely related question, but it's about a viewstate issue