I know many are thinking Robb was a screw up and got what was coming to him - and granted sending Theon north and marrying Talisa were screw ups - but I don't think people are appreciating how much Edmure Tully screwed him over by not following orders.
By denying the crossings to Tywin and his army (back towards the end of season 2) it not only kept Tywin from going west and into the trap Robb had set for him, it also allowed Tywin and his army to be close enough to return to King's Landing just in time to save it from being sacked by Stannis. And the icing on that cake was that in the process of denying the crossing, Edmure captures some low end Lannister noble kids - who Karstark then proceeds to butcher, forcing Robb to behead him and then lose all the Karstark army. So instead of Robb wiping out a good part of the Lannister army, possibly capturing/killing Tywin, having relatively unopposed access to the Lannister territory (and thus the Lannister riches), and having whoever ended up "winning" King's Landing left so depleted they'd probably listen to a peace offering from Robb establishing him as King In The North, or being easy prey for Robb's army - Robb and the northern army get wiped out. All thanks to Edmure's bumbling.
So what seemed like a throwaway line for our amusement "The gods love to reward a fool," spoken by the Blackfish in this episode, actually had a ton of poignancy, as both Edmure and Robb get their "rewards" - Robb a grisly death, Edmure a pretty new wife.
