A premature opening is never a good thing, especially when you're freeflying at faster speeds. In this case, the premie occurred while sit flying; the d-bag and lines went under his right arm as the parachute deployed, which resulted in muscle hematoma in his bicep as well as injuries to his elbow joint and rotator cuff.
This skydiver admits that he shouldn't have been freeflying with this rig. It was an older container with a worn/loose BOC pouch for the pilot chute, and the closing loop was loose because the main canopy was too small for the container.
You can avoid or reduce your chances of a premature opening by jumping freefly-friendly gear that properly fits your body and your canopy size to ensure you have a tight closing loop and tight BOC for your pilot chute.
Also, when climbing out of the plane, remember to rotate your rig inwards towards the center of the door to avoid rubbing your rig (and your closing pin, in particular) against the door frame.
In the event of a premature opening, a bungee cord on your leg straps might save your life.
For those of you who aren’t aware of the importance of a bungee cord on the leg straps: when sit flying, your leg straps can ride up near your knees. Without a bungee cord connecting the leg straps, this creates a hole you can fall through if there is an upward force on your rig.