The team took another comprehensive look at Nightfall and other similar proc-based weapon and item effects with this new information and we can confirm that certain effects cannot proc while under the effects of the Global Cooldown triggered by certain abilities. However, after extensive testing in the reference client, we can also confirm that this is true and accurate to original World of Warcraft as well.
We also discovered that there were specific provisions built into the original game to force this to be true. However, we discovered that this is true only if the spell or ability that triggered the GCD was flagged as a magic-based attack (or more specifically, spells flagged in our data with a defense category of “magical”).
This means that most physical abilities or spells that incur the GCD will not block these procs from firing while the GCD is in effect. An example of this is a Warrior’s Hamstring ability. This has a defense category of “physical”, thus you can still see Nightfall procs occur while you are on GCD following the usage of Hamstring. Alternatively, Demoralizing Shout has a defense category of “magical” and you will not see Nightfall or certain other items proc during the GCD immediately following that spell’s usage.
At this point it’s somewhat unclear if this was the original design intent or not, but at worst this was a quirk of how the original game was built, and at best this was part of the original design. Either way, this is not something that we currently consider to be a bug.
Thank you for the post about this, Smeet. Thank you as well to all of the other players who contributed to the super diligent research on this topic. We were glad to be able to revisit this and help clarify some of the inconsistencies players were experiencing around this and other proc-based items.
After reading a few responses I did want to clarify one minor thing; a spell’s “School” is not the same thing as its Defense Category. There are spells with the school of holy/shadow/frost/etc. that have a defense category of melee/physical, and spells with a school of physical that have a defense category of magical.
…and before it’s asked; no, we are not going to post a full list of which spells are in which spell school or defense categories
I’m also going to go ahead and close this thread since we’ve concluded our investigation into this issue. We very much appreciate everyone’s efforts to help us track down inconsistencies between WoW Classic and the original game.