Cannot convert type 'Microsoft.SharePoint.WebControls.DelegateControl' to 'System.Web.UI.IAttributeAccessor'

Unfortunately I have seen this error too many times. In my case it is regarding a SharePoint master page customization, and I believe it is related to editing the master page in SharePoint designer. Maybe it only occurs when you edit the page and then try saving it back to the Globals or some other folder, haven't confirmed exaclty the steps that cause it...

But the way to fix it is to open the master page in notepad and you'll see a big chunk of html that is commented out. Delete that chunk, and the master page will work properly again. [The next time it occurs I'll come back and clean up this entry with more specifics...]