I also experienced 2 complete PvP fails in the stress test due to layering.
The first was when i leveled in Northshire, and people in general started spamming “omg HORDE here in the tower!! need help etc”. I rushed over to the tower, and nothing was going on. 
Ofc i knew it was cause of my layer, which already was a huge letdown feeling cause it’s not like old WoW at all and immediately took me out of the world, realizing it’s just game mechanics reminding me what this is…a game, not a World. I got a dose of retail in Classic, nice.
Eventually i asked for an inv to the raid, and upon my invite i phased into a big army of alliance fighting a bunched up horde in the building. Just like that, i poofed into all of them.
Imagine if you’re someone who doesn’t know about the new feature “layering” until you actually play the game. And then you run into simple scenarios like this which would normally happen without any weird, out of place obstructions like that like it would have back then.
The confusion would be real. Anyway.
The second PvP fail happened in Goldshire. I specifically went there because a couple people said there was horde near the inn there, so i wanted to check it out and join the fight! … only to find nothing but crickets and maybe 4 people standing there, with no one else around the inn anywhere when i looked for the action.
Disgruntled from my first PvP fail i didn’t bother to fish for an invite again. I just was pissed i wasted my time going there, thinking i could just join the action like i normally would have been able to. But nope, i got layered. Tough luck, welcome to 2019.
All these type of situations have been confirmed to be able to happen, even with BRM PvP too. Entire raids could pass eachother and not know there’s another raid there waiting to fight.
You could even have situations where people specifically choose a layer to fight it out on. LOL!
How weird is that gonna be? The raids would need to communicate to each other cross faction over discord “Hey guise let’s kill eachother on that layer over there mkay?” before they can actually PvP in the world…
And then there’s gonna be losers and winners of course, and guess what they could do: The loser raid can hop layers before or upon ressing, completely heal and buff up again, and then poof into the old layer again shortly after to attack the winner raid, without giving the winner raid a chance to prohibit their recovery.
Amazing gameplay awaits us in Classic indeed. Everyone gets the short end of the stick, but PvPers and RPers are gonna feel it the strongest i’m certain of that.