Not against people, but against brands and companies?
The oldest grudge I have right now is 17 years and counting. Unless you count the fact that I will never buy a Chrysler because my father had a bad experience with Plymouth back in the 60s.
In 1993 American Express wronged me. Won't go into the details, but suffice it to say I will never do business with them. Ever. Ford and GM both pissed me off in the 90s, even after giving GM multiple chances to make good (my fault for being naive).
I'm a rational person. My Myers-Briggs Type Indicator is INTJ - the NT being the "rational" quartet. I am rational in almost all aspects of my life: dealing with my kids, deciding who to fire and promote at work, which charities I donate to, etc. The one exception is when I'm a consumer. Why does the iPod succeed when there are mp3 players with better storage per dollar? Why do people still buy from iTunes when DRM-free options such as Amazon MP3 exist?
Consumers are fickle. They are a challenge to understand. They are unpredictable.
Which is why it's imperative that every aspect of the customer's interaction with your brand goes well! You just don't know what sets off customers. And every small problem a customer encounters erodes trust in your brand.
Seth Godin for quite some time has talked about the new era of marketing. Consumers are in control now. Traditional marketing is ineffective at best; damaging (yes, that's right damaging) at worst. And he talked of this in the era of email and Internet message boards. "Back then," information spread virally around the web at amazing speed. Today, with twitter and facebook dominating the scene, things move even faster.
Speaking of social networks, its interesting to see how companies have responded to these tools. Those companies that are consumer-focused and known for customer excellence tend to use them well, those that have horrible customer experiences tend to struggle.
Xbox does this right - they have a twitter account (XboxSupport) that users can fire questions to. And they staff and respond to it. And when they respond anyone following them can see what they've advised.
Comcast, on the other hand, likes to respond with "Can I help you?" Can you help me? Why the frick are you asking the question? Do you really think I would say "no, I like not having internet access that I pay $60 a month for." (Yeah I would dump Comcast in a minute... if I had an alternative).
It was 10 years ago when Amazon cut it's marketing department and put the budget into free shipping. You know, an actual benefit for the customer. Why haven't more companies learned this lesson?
Ignoring your customer touchpoints isn't a smart way to do business. Some customers hold grudges a long time. 17 years. Maybe longer.