OK. It happened. Something went wrong and one of your customers is on the phone yelling. True, they should have read the fine print at the end of the user agreement before they accepted liability for using your product. But right now that is the last thing you want to tell them.
At some point in time, we all have to deal with a dissatisfied customer. Someone who waited two days past the 30-day return window. Someone who forgot to turn their cell phone off before charging it. Someone who tried to install a new sound card in their computer without grounding themselves first. At some point in time, we will have to deal with someone who is upset at us for something that wasn't our fault.
When it comes to customers, you need to keep your eye on the prize - if they can't be happy with your product, they should at least have a pleasant experience with your brand. A 30-day policy is, in reality, just a guideline. If you can still sell the sweater, accept the return. If it's out of season, ask them to bring it back when you can sell it or recommend a location where they can sell it now (like a clothing exchange).
Technology problems can be a learning experience for everyone - use this as a chance to teach good usage behaviors. Then throw in a 10% discount on an upgraded product to apologize for the inconvenience. I'm sure your margin can't handle that little bit of wiggle room. It might also turn a disgruntled customer into a lifelong client.
I said once before that the customer is NOT always right, but they should feel like it anyway. It doesn't really matter whether it was your fault for not explaining something, their fault for not reading something, or Bob's fault for not pulling a defective product off the shelf. In the end, you have someone who cares enough about your brand to raise an issue. You have someone standing at your door offering free market research on customer expectations and the overall customer experience.
Do you turn them away or do everything in your power to turn this willing consumer into your perfect customer? Even if they don't walk away with a handful of cash from a return or a replacement widget, they can still walk away with a positive brand experience. The next time this (formerly) unhappy customer talks about your brand you want them talking about how great your support services are, not how frustrated they were with contacting IT.