Conceptually related to WFCORE-1879, but here the case is a slave that is already registered handling an /extension=foo:add operation sent by the master. It's theoretically possible that the slave might be running an earlier or equal version of all extensions that were configured when it registered, but for this new "foo" extension it has a newer version. If this happens the add operation should fail. The extension cannot be configured while the domain topology is like this.
Note if the slave is ignoring the extension add op, that is fine.
It's possible this is already somewhat handled. If an extension is added, for transformation to work properly the slave has to send back management API information that the master can use going forward. It's possible though that even that is not being done, which would be a bug.