Figo Fei,
Curious that you should move my post to a "Do not post here area".
The steps are not clear and I gathered 3 top shelf developers from my group (I was the fourth) around me to witness this. I think it was only happening when I was copying and pasting code into an AjaxControlKit panel control. It would take the table which I had pasted in and changed the <tr> and <td> tags to <tr runat="server"> and <td runat="server">
While we were standing there the screen filled itself with unbreakable spaces ( ) WOW!
Previously it was also including random ID="Tr01" identifiers in the code, but when I set the Options for HTML to not Auto ID Elements for Pasted Items it stopped that. That was horrible because it was not giving me unique values.
I thought someone was playing a joke on me, but that was not the case.