An explosion may result in people reporting firearms discharge to police (this has happened, not to me).
Exposed skin close to an exploding bottle may be torn (I've heard of a cloth shirt placed over an exploding bottle being torn to pieces). Use a long hose on your pump, keep away from pressurised bottles, inspect bottles for damage before each launch. Pressures over 100psi are for serious and experienced rocketeers. Smaller bottles generally have a greater pressure capacity, and in my experience perform better too.
A falling rocket has enough energy to kill someone if it hit them right on top of their head (I've heard around 27 people get killed by falling coconuts each year in Indonesia). Parachutes are not 100% reliable, even if you use them, make sure no-one is around or they are aware and observing each launch.
A launcher that fell over could result in someone nearby being hit with a PET bottle travelling at 200km/h (125mph). Make sure your launcher can withstand gusts of wind or other events that might push it over.
If respected, PET rockets are safer than a cricket ball or other bat-using sports.