When people are angry they are anything but rational, so the amount of brand damage they can do for you is near limitless. Imagine if a person or small group has reach to a group of people entering your space, and tells them that you are unethical, a liar, worthless, etc.
If such a statement is contained then no big deal, but if it starts spreading as common knowledge people will just assume it is true. For every person creating media there are 100 people quietly consuming it, and if you are successful and have mindshare people will try to tear you down every month.
When Unprovoked Try to Be Empathetic, if That Does Not Work, Then Consider Highlighting the Issue
If they are already creating unprovoked brand damage then they are probably angering other people too. If you can't clear it up directly it might be a legitimate strategy to call them out on it such that other people they offend down the road will discover your brand. Another popular strategy is to ask friends to clear it up if you want to keep yourself removed from the conflict.
Do Not Make The Google Engineers Editors Angry
We are all flawed, and the goalposts are always moving. One day we are at the top and the next day people are surprised at the fact that we are a spammer.
One of the things that is most likely to kill a successful SEO job is boasting about the ROI and/or how easy it was. Ever since Google started aggressively editing the search results the difference between a successful strategy and an ineffective one is often one blog post. Brent Csutoras gave a lot of great examples of strategy gone awry in his STFU post.
People Inadvertently Screw You
Back around 2004-2005 Google was having issues with 302 redirect hijacking, so I made the SEO Book affiliate program use 301 redirects. I mentioned that those links passed weight in our online SEO training program. 301 redirecting affiliate links is a popular way to build link equity, but after Rand used my site as an example in the following video those 301s no longer pass PageRank.
People Trash Your Site as Spam to Justify Their Spam
Remember when Jason Calacanis was launching Mahalo, and how he started railing on about Squidoo being spam before he launched his site? A year later the truth washed out that Jason intended to create a site
with content that would be categorized as spam by Google's internal documents.
Consider Future Effects
Many years back Jill Whalen and I had a falling out because I was bidding on people's names via AdWords, and she did not like it. She thought it was scummy for me to bid on other brand names, but she had no desire to police her affiliates when they did the same. To this day she still slings mud at me, calling me a black hat, etc.
Public Online Communities Eat Their Young
Dan Thies, who wrote an ebook a couple years before me, had to battle through some nastiness as well, so I am not sure what percent of what I dealt with was natural feeding off the young or if the people complaining about me were actually mad at me. Given that they didn't mind when they profited from what they did not like, I would guess that it was mostly the former.
The big issue with eating your young is that you never know when it will come back to haunt you.
Someone Might Become a Star
Some people who get established allow their egos to grow beyond any rational limit, and are nasty to many new people entering their field. But the thing is you don't know who is going to become a star down the road, and who will have the influence to crush or embarrass you.
Consider how Shel Israel angered Loren Feldman years ago. Shel had long forgot doing so, but then Loren registered ShelIsrael.com and put up a sock puppet show that lasted for months!
That conflict just ended, but the associated brand damage will last for years. Here is Loren's take on why he did what he did:
When I first started my career, you made it a point to bury me online, and more importantly back channel as well. This is a fact. You and your crew went out of your way to take food off my plate. I never forgot that, and now you have something you’ll never forget.
Communities Are Full of Cliques
One of the things I struggle with in the SEO field is that so many of us end up doing so well that sometimes we let our egos get ahead of what made us do well and we forget where we came from. And so I hear negative stuff about interactions between many friends. Its hard to be empathetic when it seems everyone has wronged others at some point in time. I know I have screwed up more times than I can count, and much of the conflict ends up being drama for the sake of marketing.
PageRank was, is, and will always be a flawed concept. In some cases the best person wins, but in many cases the best person loses because they were not good at public relations and marketing - or because they made somebody angry, and they decided to blackball them.
Some of the top communities in the search marketing field do not get along well. Incisive Media employees and Third Door Media employees are banned from attending each other's conferences. Ever since Danny stopped doing the Search Engine Strategies conferences I have been asked to speak a grand total of 0 times. Guys like Graywolf and I were replaced by sponsored panels.