Sys.ArgumentUndefinedException: Value cannot be undefined. Parameter name: id
This was from a page that included some form fields, some validators, and was wrapped inside an ASP.NET AJAX UpdatePanel. This had all worked fine before. I had added some other things to the page so I was systematically removing them until I could find the cause of the problem. I eventually turned to a Google Search which led me to the fantastic ASP.NET forums and finally to a blog post from Matt Gibbs that explained the issue and offered a fix.
Apparently there is a problem when using validators inside an update panel that was introduced with ASP.NET AJAX RTM. Matt says that there is a fix coming in a future windows update, but for now he has a link to a set of custom validator controls that you can download, drop the compiled DLL in your bin folder, and add his provided tagMapping section to the web.config. Plenty of more details along with the download can be found in his post.