With breast implants, you are advised not to take painkillers, anti-inflammatory medication, or vitamin E for the two weeks leading up to surgery because it can greatly increase the chances of bleeding and bruising. Excessive bleeding during the procedure can be quite dangerous, and hematomas (internal bleeding at the surgery site) that form after surgery can require follow-up visits for drainage, or even the removal of the implants.
Since general anesthetic is usually used, all of the risks associated with being "put under" are also present for breast augmentation surgery. This can involve reactions to the medication including vomiting, or even a coma or death (these last two are very rare.) Not eating or drinking after midnight the night before the surgery reduces the chances of vomiting.
Scarring occurs with almost all breast implant surgeries, but some people end up with more obvious scar tissue than others, especially if the person who performed the surgery was inexperienced. Certain techniques can minimize scarring.
Botched boob jobs can result in weird looking dimples in the cleavage or breasts, or stretched skin between the implants (in the cleavage) they can also result in lumps, bumps, unattractive fluid-filled or hard pockets, and loss of feeling in the breasts or nipples.
There is always the risk of the implants becoming hard and rigid due to capsular contraction, and implants may rupture, causing the breasts to "deflate." More surgery will be required to address these issues.
Also, most breast implants do not last the lifetime of the patient, and repeated surgeries may be needed to keep up your look as you age.