This one is WR error as we have talked all week. Terrance Williams is a good player who has a few instances on occasion where he demonstrates that he might have a ceiling a bit lower than we had hoped. On this one, like many slants and comebacks, you have some rules that simply cannot be broken because the QB is trusting you. And when he trust you that means he is throwing the ball when your break foot hits the ground (because at that moment you have the position needed) and leaving the rest of the play up to you to do your job.
Once Williams does not stay inside Dominque Rodgers-Cromartie and loses the 5 yard race to the sideline - and in this case actually leaves his feet which means he has no way to help tackle the man he just lost the race against, then the play is dead - and the game might be, too.
Two other thoughts - DRC runs Williams' route better than Williams. This could be a chance to pile on #83, but it could also be a chance to suggest that they need to burn corners on tendencies more. Also, Doug Free is getting pushed back into Cassel on this throw, not helping the angle.