I think that's part of the mini-profile (now I call it the mip bug )

You can do either one of these:

Wait for Ross to sort the cache
or
Go to the person's profile, go to modify and then just save, Don't change anything, and it should be fine.

To add to what Wrighty said, changing the forum ranks or any member group will also refresh your forums cache and fix this. However from this point onwards, ranks will no longer be cached due to issues they can cause with multiple skins.