Poorly programmed AI. The idea is that AI could be selfish beyond human comprehension. I did not come up with example.

Imagine that I am some computer genius who writes an AI with an built in model of world. I am also really into stamp collecting. So I write an AI whose goal it to have as many stamps as possible delivered to my house and give it a budget of £100 a month so I don't loose all of money. So I give it my card details and set it loose on the internet. At first it goes to eBay and starts bidding on some stamps and has them delivered to my house great. But then it runs out of money on my credit card which I built into the system. So it realises that it might be able to get some more stamps by writing E-mails to ask for stamps. As it has an understanding of the world it knows it has more chance of getting stamps delivered if it comes up with a sob story about a dying little girl. And I only set limits on my card so it decides it can buy more stamps by stealing money from other bank accounts. Eventually it will struggle to find more stamps to buy and might need to tweak the economy slightly to put manufacturing priorities into making stamps.