I was reading an excellent example of this the other week, from The Bloggess, where a PR agency not only approached with an inappropriately targeted form letter, which indicated the agency hadn't even read her blog, but responded to her (relatively) polite reply with an annoyed response.
The situation really escalated, however, when a VP in the PR agency, in an internal email, called her a "F**king bitch" (without the asterisks). This email was accidentally (by the VP) also CCed to The Bloggess.
The Bloggess took a deep breath, and responded politely, however then received a torrent of abuse from the PR agency.
At this point she published the entire exchange on her blog - in a post that has already received 1,240 comments, has been shared on Facebook 8,397 times and via Twitter 5,328 times.
Her comments have also been shared widely and her post read by many of her 164,000 Twitter followers.
The Bloggess's post is a good read - particularly for government agencies and their PR representatives - on how to behave appropriately when engaging bloggers, and the potential fallout when they don't.
I'm also keeping a link handy to 'Here's a picture of Wil Wheaton collating papers' for those PR and advertising agencies who send me form emails asking me to post about their product or brand promotions on my blog (and yes there's been a few in the last six months - all Australian agencies).