It is possible to have a negative reaction to the anesthesia, excessive bleeding, infection, hematoma and seroma. Of course, it is given that scars will be apparent that are associated with Mastopexy. Permanent loss of sensitivity in the nipple area and breast skin is possible. If you smoke your risks are increased not to mention that your scars will heal slower and possibly wider than a non-smoker's would.
A big fear is tissue necrosis (tissue death). I am serious, you do NOT want this. Tissue Necrosis happens when either you smoke and you have poor oxygen-tissue saturation or the surgeon did not use a pedicle to keep blood flowing to your nipple or other skin sections that were reattached. It also could be just bad healing. This is an issue and by far the most worrisome and dangerous so do all that you can to keep this from happening -- like stop smoking several weeks beforehand!
Infections, although rare can happen due to bacteria such as Staph, which naturally lives on your skin. That is why it is important to wash your breasts, neck and torso with an anti-bacterial soap like Hibiclens or even Dial anti-bacterial soap for several days up until your surgery. This can reduce the amount of Staph on your skin.