I understand the importance of creating communities on various StackExchange sites that are tightly organised around a specific topic and keeping the questions on each site on topic. However, I believe that at times there are questions the overlap domains that might be appropriate on multiple sites.
For example, this question about power for Apple computers when traveling was migrated to superuser.com. While it certainly fits within the scope or superuser.com, I think it was a perfectly valid travel question. And if it did need to be migrated, apple.stackexchange.com might be a better fit than superuser.com.
This is really a broader StackExchange question which I don't know if it has been addressed by Jeff.