When attempting to change the series detail for mailman-administrivia/main I wanted to set the branch to ~mailman-administrivia/mailman/admin, however that branch is already associated with the mailman product/project. The error message you get is:
~mailman-administrivia/mailman/admin is not a branch of GNU Mailman Administrivia.
The problem is that it's incredibly difficult to fix this because for one thing, it's not obvious that a branch can only be associated with one product (unless you know the code <wink>), and for another, the error message does not tell you which product the branch already /is/ associated with.
At the very least, the error message should include the branch.product name, with a link so that you can go to the project the branch is already associated with.
This may be related to bug 31308 because it would be nice if the error message pointed you right at the u/i to change a branch's product.