I mean, the risk of injury partly depends on how early is "early"....from just taxiing off of the runway at one extreme to the other end where the plane is slowly creeping the last few feet into the gate parking spot. But you'll occasionally see more than just early unbuckling...
Last year flying on a few domestic flights in Greece, every time, most passengers stood up and started grabbing their bags and rushing to the front of the plane probably about 3 feet before we came to a stop at the gate. In one instance, my wife and I were in aisle and middle seats and the guy in the window seat literally started trying to climb over us right when this premature mad rush started (nevermind that the aisle was already packed full).
A few years back on a flight CNS-SYD, there was a Chinese tour group (about 30 people) onboard. The FAs had a hard time getting a few of them to stay seated when we were close to final approach. Then as soon as the nose wheel touched down, most of the tour group stood up and began retrieving their bags from the overhead. This precipitated a PA announcement from an FA - which did no good. We stopped for a few minutes on the taxiway immediately after clearing the runway and the FAs had to come back and get them to re-stow their bags and sit down.