Okay, so I know a lot (if not all) of us are freaking out with those 2x15 stills, me included, and like a lot of you pointed out looks like they are wearing the same outfit from 1x12, meaning this could be a flashback.
BUT, I’m a piece of shit and I had to be 100% sure those were the same outfits and here is what I noticed:
Magnus is wearing the same robe, but in 1x12, he was also wearing another shirt under it. The necklaces and rings are different too. Plus on the new still he has a white streak on his hair that he didn’t have in 1x12. (Also in the other stills that he’s with the red shirt he’s wearing the same necklace and the make up is the same from the blue robe still.)
Now Alec. His shirt has the same color, but the collar of them are different, so it’s not the exact same shirt.
My conclusion is: I have more reasons to believe this is not a flashback scene and just a coincidence than the opposite. Yes, Todd said we would have a Magnus flashback from season 1 and that would involve Alec, but that doesn’t necessarily mean that Alec will be in the flashback. He could be talking about him. Also, I don’t believe they would be this sloppy and make such a stupid continuity mistake.
Another thing to point out is that by 1x12 they didn’t have the intimacy that they’re showing on the stills. I think it’s also worth mentioning that last episode (2x13) Alec was wearing the same shirt he wore in 1x04, so they could be doing the same thing with Magnus. And, if you think about it, Alec is always wearing the same type of clothes *3 different green sweaters this season*, so it only makes sense he has 5 different shirts with the same color.
Anyways this is all speculation and I could be 100% right or 100% wrong. Let’s just calm down with the theories and see what happens. Be safe people! :)