You want to know how bad it can break down? I imagine the worst case scenario, for so many reasons, actually happened and was mtgox.com. It started out as a Magic: The Gathering Online Exchange (from what I understand) before it became the now infamous Bitcoin exchange that was hacked[1] and massive amounts of money was stolen. I don't know for a fact that old accounts before the pivot to Bitcoin still existed and worked, but it's not inconceivable that they would. One hopes they adopted much better security compared to when they were a trading card exchange (if it wasn't already exceptional at that time), but there could very well have been a time when it was gaining traction for financial type services but didn't have good account safeguards.
1: Or whatever. From what I remember that's a story convoluted and with enough conspiracy theories it's worth a movie.