I know mine tips (but I looked at it from an engineer's viewpoint and it made sense) because I keep the power brick in the front pocket. Pretty much whenever I have my laptop out and using it, the bag'll fall forward, but I simply place it a little farther away from a wall and lean it back (but it takes an angle of about 15 deg or more).
Now if only I could become graceful enough to put the strap over my head without disturbing my glasses, I'd be set!
I only care about the tipping because when I am waiting for my bus or waiting in line for something, I set my bag on the ground. It's not that big of a deal, I was just curious. I have a brain cell and laptop whenever I use either bag but I guess since the laptop weighs 3.5 pounds, it doesn't give much counterweight.
When I'm using the Smart Alec backpack, though, I don't use the top pockets while I do have a binder or books in it and still tips over.
Not a biggie, but it is a minor annoyance. And if it didn't tip, that would serve as an excuse to my husband to buy it. He likes to make cracks about how I have most of the TB bags (when I pointed out that I "only" had 5 of them.....)