I suspect neither Coke or Mentos planned for the tsunami of videos on Utube and others demonstrating the effect of a Mentos in a bottle of coke. Nevertheless, it happens, and it remains to be seen if the popularity of coke-bombs impacts the brands in any way.
Blend-it has made a business by demonstrating the blending power of their appliances by blending all sorts of things, latest is an Ipad, but this by contrast is a deliberate marketing strategy that has delivered a huge brand position for little cost.
The point is that the power of the web can be harnessed, and used to your benefit, but it is a demanding, unpredictable mistress, and just as prone to turn around and bite your bum, as it is to do you a favour.
Believing you can manage the web content that impacts on your products is the first mistake, best you can do is participate, contribute, comment, and if you do it well, as Blendtec has, you can leverage the power, never control it.