Not sure about the penalty decision since it's not on the highlights, but if Forster didn't spend the whole game diving and whining at the ref then there'd have been less of an element of doubt.
Never a penalty (though I've seen them given). He put the ball past Sheehan (maybe through his legs) and then ran in to him. I don't think there were any legs sticking out, more like Forster body checked Sheehan then collapsed looking for a penalty. Even obstruction would have been harsh as Sheehan didn't appear to move.
I'd hate to hear Poyet after a game where they did actually get some bad decisions.