I have noticed that if I do NOT have a user in my roster, but have added them to my blocked user list using the Yahoo client, they will appear when fetching the roster in my Jabber client.
0) Create two Yahoo IM accounts with empty rosters. I'm calling them "user A" and "user B".
1) Log in as user A using the Yahoo official client
2) Log in as user B using the Yahoo official client on another machine
3) Send a message from user A to user B
4) Add user A to user B's ignore list by clicking on the "Ignore" button in the Yahoo chat window of user B
5) Log in to the gateway as user B
6) Notice that user A appears in the roster of user B even though it was never added to it at any time
I am assuming that Yahoo sends a list of "blocked" contacts and the gateway is interpreting them as roster contacts. Is this the case?