This is in the newest Classic version.

When trying to sort bags while having a relic coffer key in your bags after keyrings was added, the coffer key will repeatedly try to get sorted into the keyring which is impossible since it doesn't go there, causing an endless loop of "This item doesn't go in that slot" error messages requiring a ui-reload to stop it. Deleting/removing the coffer key from my bags makes the problem go away.