Customer complaints are annoying and irritating. If you're on the supply side of doing business, complaints are like your staple food. Here's some of responses I recommend for you guys facing the wrath of customers:
Blame other department.
Blaming other division of your business is good. It projects the image that you're the competent star among your company's pool of slackers. "If I'm the one assembling this product for you, I'll make sure you're smiling to your ears".
"Sorry this happens to you, I'll give them a good shove in the butt so they'll remember this mistake" It'll give them a better peace of mind. A fake one.
Blame the top management.
This is a different trick from above, where you project yourself as a sorry victim of your company's ruthless management.
"We've told them this feature will be disastrous, but they won't listen. At the end of the day, we're just a wimp trying to make a living here". Sympathy will soften their anger a bit.
Blame the uniqueness of the case.
"I say, we've never encountered this situation before. Yours is very unique. We need more time to rectify this." will actually make the complainer feel more exclusive, as if the whole company is now working hard just for his lone whining. May even result to higher satisfaction.
Blame own inexperience.
This is an old classic. "I'm sorry, I am new here." Of course they'll blame your company for fielding such noob in the line, but hey. As long as you're safe, it's all right. Right?
Blame the customer.
This is a perfect mirror. It deflects the ray back to them. "Sir, with all due respect, we think you've done something that jeopardized the product". Of course they'll be more defensive than ever, but your company is the one producing those product/services, you should know better about them.
Create something that's even remotely possible, get some ideas by interviewing the complainer. There's always common mistakes around. Use them. Like how auto insurance companies use in the case of a theft.
"If you sleep in your car, this wouldn't happen. This is negligence on your part. Your fault."
And quit your job. Because there's bozos who just like to complain, even for trivial things. Things that can be solved if they take half a second and think.