Financial infidelity is one of those common things that we've only just started learning how to talk about. When you have a joint account, you need to have a lot of discussions about how that account gets used, and just what you're spending money on. Even if the relationship has a foundation of trust in other areas, money can be different. Everyone has a different relationship to it, and not everyone agrees on how it should be spent. But even if you do have all the necessary conversations, it won't stop duplicitous people from going behind your back and spending money in ways you don't approve of.
That's exactly what happened in this recent story from Reddit where a husband went behind his wife's back in a pretty major way. The comment section then became a hotbed for discourse and advice for this woman who had been hugely blindsided by someone she thought she could trust.