I was trying to setup a trust between two domains across ISA2004. Everything running in Virtual PC. But it did not work. Whenever I tried, I got the message The operation can not be performed on the current domain. I was not able to find this message anywhere on the Internet and the message gave no hint as to why it wasn't possible. Trying to explain the problem to my colleague, I realized the problem myself: I had copied the original Windows Server 2003 disk (file) in a workgroup state. DCPromo seems to reuse the SID of the original server for the domain, so the problem was caused by the fact that both Active Directory domains were having the same SID. Hmmm - I knew that I should not just copy the disk, but a decent error messages would have helped a lot. Demote, sysprep and promote removed the problem.
If you by chance should experience the same situation on NT4, the error is also a strange one. As I'm not 100% sure, I'm not going to provide the message but it had something to do with file/record already exists.