This is true when you're trying to do 2 casts in the same macro. It also fails for some (but not all) trinkets for some weird reason.

i.e. (and I apologize for the crazyness of this macro, but this is honestly my button spam macro)
/target [target=focustarget,harm,nodead][target=party1target,harm,nodead]
/castsequence [combat] Vengeance of the Illidari, Ancient Draenei Arcane Relic
/castsequence [nocombat,harm] reset=5/combat Totem of Wrath,Lightning Bolt,null; Lightning Bolt
The code above works perfectly for me. The code below does not:
/target [target=focustarget,harm,nodead][target=party1target,harm,nodead]
/use [combat] Vengeance of the Illidari
/use [combat] Ancient Draenei Arcane Relic
/castsequence [nocombat,harm] reset=5/combat Totem of Wrath,Lightning Bolt,null; Lightning Bolt
The 2nd macro will "stick" when Ancient Draenei Arcane Relic is on cooldown (I've deduced that it's due to a "fake" GCD), and refuse to work until it comes off of cooldown.



However, the macro you showed me has one spell and one spell only in it: Frost Nova. Therefore, it will not trigger the GCD.

Another example test you can try at home: make a macro that just says /cast Frost Nova. Button mash the macro. No GCD when Frost Nova is on cooldown. (again, I used "Holy Shield" since I don't have any mages)