I was doing the last quest of the companions and on the way to the crypt(I dont remember the name) a dark brotherhood assasin attacked me. I already completed the darkbrotherhood questline, and astrid died but on the note from the assasin it says astrid. does anyone know why and how and if it matters?

It doesn't matter. This has happened to me as well. I've always assumed it was an old note and an assassin that was away a long time and doesn't know all that has gone on.

It is a http://www.uesp.net/wiki/Skyrim:World_Interactions

Sure, but aren't those attacks supposed to stop after you complete "Innocence Lost"?

Unless it was triggered prior to finishing that quest.

I don't know, just to be safe, maybe go back and kill them all again...

Nothing triggers it, though, they're random encounters and, yes, they shouldn't be happening after Innocence Lost, much less after completing the whole questline, presumably something's bugging out in OP's game.

+1 Valandril. As far as I know....each world encounter is fresh. Meaning you can't have an old, previously triggered, assassin kinda waiting in the bushes miles across the map just waiting for you to happen by. When you reach a WEP the game chooses a random npc for you. After innocence lost, the Assassin should no longer be on the list of available npcs.

Having said that.....I have come across Balbus in the same place several times even though I had been well outside the cell in between.....like he just refuses to go!

My guess..... Valandril nailed it....some screwed up piece of code in there.

I've often thought.....although I take very good care of my discs........if you had the tiniest flake of impurity on the disc surface.......that's gotta be obscuring code right? Probably nothing....like a pixelated dropout on a random piece of scenery right? But....it COULD just as easily obscure vital code that enables a quest.....or switches off an assassin.....just a thought!