The specified boundary already exists!

You try to add a boundary that you are sure you had added before and you get this error:

That's because you were right! You had added it before. But it just disappeared from the console, but not from the sitectrl.ct0 file.

Resolution:

Go to the SCCM console, go way down to the end and expand Tools. Right click on ConfigMgr Service Manager and select Start ConfigMgr Service Manager.

Was the boundary created in the primary site? If yes, then just expand the Primary Site Code, then expand Componets and click on SMS_EXECUTIVE. Then in the right side right click the SMS_EXECUTIVE and click Query. Right click again and select Stop.

Now browse to SERVERNAME\c$\Program Files (x86)\Microsoft Configuration Manager\inboxes\sitectrl.box and open sitectrl.ct0 using notepad.

Now locate the BEGIN_ROAMING_BOUNDARY and remove the missing boundary. Save the file. Go back to the console and restart the SMS_EXECUTIVE service.

If it was a secondary site then click on site on top of the window and select Connect. Type the Secondary Site Name. Then follow the same steps as above.

Now you should be able to add the boundary again.