If you sell items on eBay or own a business and take payments via PayPal or Credit Card etc. these can be charged back many months down the road.
The customer is "always right"type of thing plays out where the user can simply call or open a ticket and let them know that their credit card or PayPal account was stolen or hacked into and that they did not make the payment. 99% of the time, they will side with the customer and refund the user back the money they spent and you the seller will lose out on those funds. They usually have up to 6 months after a purchase to make these disputes so if you sold physical goods or sold bitcoins, you are at risk for up to 6 months.
The reason why most buyers get away with false chargebacks is because the seller is usually too busy to respond to the claim or they don't notice that a buyer made a claim. I had a buyer on eBay who actually had their account hacked so they made a claim and I responded back. PayPal sided with me and I got to keep the money but I still gave him a refund.