It's not so much out of focus ... it is just that you have focussed on the front twigs with a shallow depth of field. Also, as turtle
mentions, the telephoto range will lessen the DOF too. I wouldn't necessarily say you have run into the limitations of the zoom just yet.
What you should do is go take that shot again. You have a digital camera - take a few shots. Do some bracketing. Stop the lens down to f/16 or more and see if that makes a difference. It won't cost you to develop the shots.
Btw, I have the 50mm f/1.4, and whilst I like it for indoor and low light photography the autofocus fishes a bit. When you get it right, though, it's wonderful. But you have to be careful - especially if using it wide open at f/1.4. For example, at that shallow depth of field, if your subject's face is not front on you will only get one eye in focus.
The f/1.4 also has better construction and the 8 blades provides better background blur, which is an effect I like.