
I got one of these "leakproof" bottles the other day (thanks, ironNEO!) finally figured out a way to get it to leak.
These have a unique valve on the top that creates enough of a seal that it will keep liquids in, at least most of the time.
It was sitting on my desk, loaded with bright green smoothie when something weird happened. The air inside it had warmed and expanded enough to create some pressure inside it and it "burped". The sudden release of air also carried with it a fine shower of green liquid that misted down on my computer screen and white shirt.
You might not think this would happen all that often, but the irony is that this is an insulated bottle; you are encouraged to put in cold beverages that will eventually warm and expand.
I think if you used the bottle on a bike while riding, this really isn't an issue and the bottle is a great device. My one real complaint is that they made the lid (which is the real heart of the purchase) threaded differently than the standard bottles you already have. Want to put the lid on a different bottle? Sorry, you've been cock-blocked with proprietary B.S. methodology.