If you use <OBJECT> tag to download assemblies and embed them into HTML document you should download and use SP 1 of .Net CLR. Due to a bug in mscorie.dll, this is part of the .Net download mechanism. It appears that sometimes certain or all assemblies simply not being download to the client. This behavior cause the user to work with unappeasable version of the software without any indication to the user that he is working with the wrong version or at least that something has been failed.