From the Angry ofIslington blog :
"A few Arsenal players questioned the linesman while Cole was being treated, and the linesman certainly had a better chance of seeing it than the ref. But from the linesman’s angle (facing Cole directly) at full speed, when maybe he was concentrating on his main jobs of seeing who the ball last came off and whether anyone was offside, I can see why he missed it. Maybe he shouldn’t have missed it, but it’s feasible that he did. You may not believe me about the speed. All I can say is go and watch it again yourself."