Brow drop and uneven brows are two outcomes a forehead treatment can leave behind. Both can surface after the patient has left your chair.
What the review documents
PubMed Central hosts a review of upper face toxin complications. On the first problem it states: "Eyebrow ptosis is a common complication arising in the treatment of the frontalis muscle for addressing horizontal forehead lines with botulinum toxin."
On the second, it states: "Another complication that has been reported is eyebrow asymmetry, which can result from the site and amount of application difference or anatomical variations in the patient."
The second sentence names two causes. One is under your control and the other is in the patient's anatomy.
Product and condition references
For product information, MedlinePlus has drug information on prabotulinumtoxinA-xvfs injection. Background on the condition behind the toxin is at MedlinePlus on botulism.
A brow note at every first visit
Look at the brow and forehead before you inject and record any difference you see. A trainee in Parsippany, New Jersey can build that pre-treatment note into every first visit.
The American Society of Plastic Surgeons page on botulinum toxin is a patient-facing reference worth keeping at hand.