In a letter to House Speaker John Boehner, Treasury Secretary Jacob Lew projected that the department’s “extraordinary measures” currently being undertaken to avoid default will be “exhausted in the middle of October.” From there, Lew writes, the United States would only be able to be funded by whatever cash Treasury has on hand, estimated to be about $50 billion. Lew calls that potential situation “unacceptable.” …

That’s because Treasury doesn’t get to pick a date for default but rather is subject to the ebb and flow of government revenues and expenditures. And those figures are anything but predictable, as how much the government owes its creditors on a given day—and how much cash it has to pay them—is set by a host of volatile economic, legal, and political factors.

“It’s very difficult to tell, particularly this far out,” when exactly the Treasury would have to default on its debts, said Steve Bell of the Bipartisan Policy Center. “October is an extremely lumpy month. Some days there’s cash coming in; other days there’s cash going out.”

And that unpredictability makes an accidental default all the more likely, even if neither side wants it to happen, Bell said.