The case where I failed one great customer long time ago, was delivering them fast ADSL connectivity. I agreed with them, to configure their system to use the ADSL. But when I were there we found out that the cabling to the location was so bad that the ADSL was barely working and wasn't reliable. I failed, I told the customer to complain to the ISP. I should have take care of all that, even if it takes forever on queue and so on. As well as it might require tracking and reporting for weeks, several visits. So it's dreadful, but I should have done it. I lost the great customer due to this. It's important to place customers in right segments and do proper customer segmentation.
These are questions which pop up in IT support, ICT projects & software development up all the time. Is this really a thing we're willing to make? Is it our job or not? It's easy to say no, but if the customer is right, it might be a good idea to bend a little.
My major fail and thing learned since that, was that I didn't recognize the best class of customers. The customers, who pay basically what ever you date to ask, but then you'll need to do almost whatever they ask. At times it could be daunting, but it also means good money. You shouldn't ever say to them that you'll need to take care of this. Right way is to prepare mandate and just ask them to sign it. Then you'll take care of the things and invoice for it. They're happy, you're happy.
I've noticed that many businesses do not properly segment their customers. They either provide some service, or they don't. But that really should be done according to the customer preferences an level.In past I've learned that in some cases, it's absolutely vital to give best service you can. Some customers just want that everything works, and it's ok that it costs. Other customer segments are more price sensitive and might not want to buy all services from you. Of course then there's the worst class of customers which shouldn't be maybe customers at all. They want everything, but for free. It's not a good business.