What does "queue other spells" mean here? Basically you can queue a spell at most 400 ms before it becomes ready. For example, you can queue the next spell before the current spell finishes casting; or if the next spell is in cooldown, you can queue it 400 ms before it completely cools down (this include GCD).
This spell queue works fine, until you are also queuing spells that works with your next weapon swing (heroic strike and cleave. i haven't tested DK/Druid yet, but it could also be the case). If HS/Cleave is in queue, then there won't be any queue for other spells, say bloodthirst. So instead of putting bloodthirst in a short queue, the game will try to cast the bloodthirst immediately. Then of course it would fail, because the server knows the spell is not ready yet.
I have made a video recording and posted it on Blizz forums -> bug report, but got no response whatsoever. So what do you think?