Recently I found that a contact appeared to be blocked, even though it really wasn't and we could see and talk to each other. To be exact, the status icon was a red x and the block/unblock toolbar button had a green v (unblock icon) but was inoperative (enabled but didn't do anything), but in the contextual menu of the contact list there was a working "Block…" option.
When I investigated I found how this happened. Normally I keep my privacy level at "Allow anyone", but sometimes I switch it to "Allow only certain contacts". Before this happened I switched to "Allow only certain contacts" momentarily and removed the contact that subsequently appeared to be blocked (inconsistent with other contacts that have never been on the "Allow only certain contacts" list; they didn't appear to be blocked). When I switched back to "Allow anyone" that contact still appeared to be blocked. This also 'works' when I switch to "Block certain contacts" and add someone, then switch to "Allow anyone", and presumably more combinations.
Initially noticed with a .Mac account (configured as such, not as AIM with @mac.com suffix) and AIM contact, but now also tested with GTalk and MSN; seems ubiquitous.
Not sure if "Adium UI" is right; change as needed.
