If you're working with territories by using the SDK, you might get this error message:
0x80043805: Territory manager cannot belong to other territory
This happens if you want to make a specific user manager of two territories. This is possible, but you will have to make sure that the field 'territory' is set to NULL of this user. When you change the manager field of a territory by using the SDK, then the attribute territory of the user is changed as well. If you want to set this user as manager of the second territory, then you'd first need to clear this field.
Once again, I hope this saves you some time!