Error in Calendar Extender because the master page has asp tag <%%>

Today, I just annoyed with this error.
Every form which using Calendar extension works fine before.
and somehow after I change the master page, it start to show an error said :

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System.Web.HttpException: The Controls collection cannot be modified because the control contains code blocks (i.e. ).
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Luckily, with Tom's Help, I found the problem.
It was on the master page on the header java script which use <%= ... %> to get the content from server....