Patrick McKenzie had a great horror story on his blog a couple of years back. He runs a service that provides appointment reminders to businesses' clients (e.g. "Don't forget, you have an appointment to get your hair colored at Best Little Hair House tomorrow at 3"). Long story short, an attempt to manually correct a hangup in the live system resulted in his product spamming his customers' clients (that's right — not just his customers, but their customers) with up to 40 phone calls back-to-back.

So, how many customers do you think he lost because of this? The answer is two, and one of them signed back up because they were impressed by the great job he did in handling the fiasco.

Moral of the story: As long as you really are making your best effort, you might be surprised how willing people are to deal with human error. Yes, they might be be mad, but a mistake is (usually) not the end of the world.