I actually took a fall almost two weeks ago with a growler in my messenger bag. I ride my bike to New Belgium Brewery once or twice a week. I was on my way home and took a right hand corner at speed just like I normally do. The problem was I didn't see how much dust was on the road left over from gravel that had been laid down all winter. As I leaned into the corner my bike lost all traction and went out from underneath me. I was fine, but shaken up enough that I forgot about the beer. I walked a block to a place I was going to pick up dinner and locked my bike up. As I bent over to lock my bike I heard something pour out of my bag. I look down and notice a nice puddle of beer on the ground.... Fortunately, the glass was intact, but the seal on the cap had broken and about half my growler had emptied into my bag (waterproof means liquid cant get out very easier either ) I keep a towel in my bag to make carrying the bottle a little more comfortable, which may have been enough to keep the bottle from breaking, and it soaked up the beer that was in my bag.

Honestly, I will continue to carry my growler in my bag. Even if it were to break, I couldn't see the glass puncturing the bag..it's pretty tough. I would be afraid that a growler in a pannier would get super shaken up since it will see a lot more road vibration than if it were in a bag on my back.