Lets cut the fluff and get straight to the point - Brand redefinition is certainly a significant possibility, considering that marketers are no longer the only people capable of creating communication which reach the masses.
The more difficult question is when can it be termed as dangerous.
The way I see it, ultimately a brand exists in the mind of its consumers. Different segments take away different experiences from your brand and your brand could mean something unique to each of them. It could be positive or negative, but on the whole it is probably different from what you intended.
Social media only amplifies this existing opinion & brings it to light, so in that sense it is free market research.
With social media a crazy fan could end up portraying your brand in a different light. Similarly an angry consumer can really create havoc especially if his post appears on the first page of google.
The key task of a brand manager is to understand when to interfere and when to let your consumers ride you.
It is more than advisable to have a sense of humour as a brand at this stage, learn to laugh at yourself if the social media adventures of your consumers are not blatantly malilcious.
"There is no such thing as bad PR" still applies in the above case.
Of course, when consumers lash out online, you probably don't have a choice but to step in and address the issue to the best of your ability. Resolving issues publicly is better if you are confident that you can actually resolve the issue, otherwise try to take the conversation offline.
What do you think? Should you interfere in the social media adventures of your consumers or should you lie back and let them shout