What are the possible complications and side effects of a brow lift?

The types of complications that are possible are directly a result of which browlift technique is chosen.
If you have a coronal lift, the chance is 100% that you will lose sensation from the deep branch of the supraorbital nerve. Hair loss and numbness are common.
If you have an endoscopic forehead / midface lift, there is a higher chance of injuring the nerve to the forehead (frontal branch).
If you have a hairline browlift, there is a higher chance for visible scars.
If you have a behind the hairline lift, or an endoscopic lift, the chance of baldness around the incisions is higher, and so on.
No browlift is right for every patient, and they all come with a series of pros and cons.
You may wish to read our book chapter on browlifting techniques in the textbook Mathes.

Many of the risks associated with brow lifting are similar to other surgical procedures, but there are a few unique risks involved. Bleeding, scarring, infection, numbness, nerve weakness, bruising, swelling, pain, and hair loss are all possible, but most of these are uncommon. Bruising, swelling, short-term discomfort, and temporary numbness are all to be expected. The degree to which each of these occurs for each patient will vary depending on the patient and the particular brow lifting technique used by the surgeon.All the best

Complications associated with
browlift surgery are fortunately uncommon.  When complications occur, they are often related to the type
of browlift incision used.

When an open approach is utilized,
problems with numbness hair loss and scarring can occur.  When endoscopic browlift is performed,
the nerves that elevate the brow can be damaged.  Both of these procedures are associated with elevation of
the hairline as well.  Hairline
incisions avoid this problem, but may also be associated with noticeable
scarring.

With any surgical procedure,
infection, bleeding, hematoma formation and swelling can also occur.

It’s important to understand that
no two patients are the same. 
Complications with browlift surgery can be minimized by carefully
selecting the appropriate incision for each individual patient.  This
individualized approach has resulted in a low complication rate for browlift
surgery.

That depends upon the type performed. Endoscopic browlifts generally have a very low risk of complications, but with all these procedures there is a risk of some localized hair loss, elevation of the hairline, scarring within the hairline, and numbness behind the incision (highly unlikely with the endoscopic approach).

There are many potential complications that can occur from browlift surgery. The technique with which the surgery is performed and the extent and location of the incisions will have something to do with the frequency of the complications. Your surgeons familiarity with the anatomy will also impact which complications can occur.
The list of any surgical complication will be included in your pre-operative discussion with your surgeon. However, the biggest potential complications that I would pay most attention to are injury to the facial nerve resulting in forehead muscle weakness and injury to the sensory nerves of the forehead resulting in forehead and scalp numbness. Other than these potential complications most of the other issues are generally temporary, of little impact to the ultimate surgical result and can be generally be corrected.

There are many methods for perfoming a browlift or forehead lift including an endoscopic browlift (through tiny incisions with the assistance of telescopes) , bicoronal forehead lift (extensive incision from ear to ear across the top of the head) and a tricophytic or hairline browlift. (hidden along the hairline. Each of these techniques achieves a similar result through the brow and the forehead in terms of smoothing of lines and gentle elevation of the brows.
The key difference is in how they affect your hariline. The Bicoronal (ear to ear) browlift elevates or pulls your hairline back, raising the frontal hairline by up to one inch. The Endoscopic lift has minimal effect of the position of the hairline. The tricophytic or hairline browlift can be used to adjust the hairline, usually lowering it to address a receding hairline or widow's peaks in the temporal hairline.