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

This error came because, in my master page there was a java script function like below

document.getElementById("<%=lblShoppingCart.ClientID %>").style.color = "#6CA2DE"

I used <%# instead of using <%=. It's work fine.

Here some valuable links related to this topic.

http://leedumond.com/blog/the-controls-collection-cannot-be-modified-because-the-control-contains-code-blocks/

http://www.west-wind.com/weblog/posts/2006/May/27/The-Controls-collection-cannot-be-modified-because-the-control-contains-code-blocks-ie-