When someone's visiting your island, you get a small portrait of them under your own on the left side. The bug is that the portrait doesn't seem to go away properly. The time I noticed it was when someone visited me, and then logged out while visiting me. I saw their name no longer had the green "online" orb in the friends list at the bottom, but their portrait was still there on the left.

A minor bug, but still something to tidy up.