I was doing a Numbers Job that took me to Candlehearth Hall in Windhelm. I fast traveled there and got caught up in the Blood On The Ice quest (I killed Calixto per the quest, but he doesn't reside in Candlehearth or anything) and then went to Candlehearth to change the books. On the way back to Riften I made a few stops, none of which I think would cause this. I stopped by a cave to kill the Skooma suppliers, and I stopped by Northwind Summit to kill the dragon (and a few skeletons in the mine you use to ascend to the summit) and Discover The Source Of Power, and I cleared Redbelly mine of spiders. However, when I arrive back in Riften to collect the payment for the numbers job, Delvin is gives me messages like "This Ain't The Dark Brotherhood", and various other messages, all of which sum up to him saying I killed the mark and then I get the message saying I failed the numbers job.
I've restarted and just tried to talk to him again, but he still fails me. Given how much time I spent between changing the books and going back to Riften (probably an hour and half to two hours of real time play) I don't really want to load a save back that far. I did try a Google search, but didn't come up with this particular problem. What's the deal?