I've had two mishaps with this bag in the 1st 500 miles of use. Both happened on dirt.
Mishap 1--I zipped the zipper so that each of the sliders was halfway around and met in the middle toward the rear of the bike. As I bounced along on dirt, the zippers unzipped themselves until my water bladder bounced right out of the bag and hit me in the middle of my back going over some washboard. 1-liter fuel bottle also self-ejected. Found the bladder but never did find the fuel bottle. Solution is to keep one slider all the way zipped and the other one all the way unzipped. This seems to be working.
Mishap 2--Washboard then a big pothole and the entire bag separated from the bike and ended up in the mud. I was using it to carry 2 liters of fuel, two liters of water, and my tools. Total weight--22 pounds. This happened on two different trips, so not a fluke. Apparently, if there's enough weight in the bag and the up/down motion is severe enough, the Velcro straps simply let go! Solution is to reduce the weight when going off asphalt. Not exactly the solution I want. If it won't carry the stuff I need it to carry, how is that a good deal for me? Maybe my new large luggage plate will work better keeping the bag attached to the bike. (Edit 6 months later: the ejections only happened with the small TT plate. No ejections after I switched to my large tail plate.)