The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

var objCmbValidateMe = document.getElementById('<%= ComboBox1.ClientID %>’)
When I sued the above code in my aspx page I was getting the eroor as .

“The Controls collection cannot be modified because the control contains code blocks (i.e. ).”
This error can be solved by just replacing the code as below .

Other wise pass the control’s clientId as a parameter to the javascript function from thes server side code.

Replace your code as below will solve the issue. var objCmbValidateMe = document.getElementById('<#= ComboBox1.ClientID #>’)

You can also find other solution in the bleow link.
http://www.west-wind.com/weblog/posts/5758.aspx