I created 2 AD Sync machine. Some day, the machine which is RID role crashed. the error "the current fsmo could not be contacted ...." pop up when I used change RID role with UI. After search online, I got following steps to fix it:
1. open cmd.exe
2. type "netdom query fsmo" to see fsmo role assigment status
3. type "netdsutil"
4. type "roles"
5. type "connections"
6. type "connect to server adservername"
7. type "q"
8. type "reize rid master"
Please refer to : http://support.microsoft.com/kb/255504/
When you tried to add computer into this AD, you could get error message:"Windows cannot create the object because the Directory Service was unable to allocate a relative identifier". This is because these 2 AD doesnot sync RID. Please refer to :http://support.microsoft.com/kb/822053