Yeah, unfortunately I can see how this kind of thing can happen in a situation like that, even without any malicious intent (at least at first)

Bill comes in slightly above what you were expecting, normally you'd go overdrawn and stuff, but it's ok - you have all this extra money set aside! You'll just remember to put a bit more aside next month.

Whoops, something broke and you don't have the money for it! Well... Surely it wouldn't hurt anyone to use some of this money for it? You'll pay it back before it becomes an issue!

And so on, until you just get used to spending this money that isn't yours, lending it out, etc, until you suddenly realise there's none left.