I may have found a potentially nasty bug with fire nova on the current PTR build (19658). The bug occurs when attempting to use fire nova right around when it is coming off cooldown. Normally the spell would simply be queued and then cast when it comes off cooldown. Instead what's happening is it's resulting in a spell not ready error until some given time after it comes off cooldown. I even had one case with the new echo where by spamming the key I was unable to cast until I had gained a whole additional charge, waiting two whole CDs to cast once it recharged. This bug DOES NOT occur with the GCD however.
I can't link to the post since my account is new but you can find my bug report here: us.battle.net/wow/en/forum/topic/16411691461
This bug could have already been fixed internally but I have no way of knowing. I'm posting here to try and get more eyes on the problem to see if I missed anything as well as raise awareness so hopefully the Devs don't miss it. I'd appreciate if anyone could help verify the bug and possibly compare it to how fire nova currently works on live better than I did. If you have a twitter account you might toss a link of the bug report to WarcraftDevs.
I think it's likely that this bug won't be fixed for the initial 6.1 release but post release hotfix based on how late it was found. I know this could potentially suck if a fix takes too long but I hope we can all remain calm and get it fixed.