So one of them snags the source from Highrise and changes some images around. I'm sure he previewed it, it looked fine and he published it. He probably forgot to change out the button images because they looked fine.
Who knows if this is how it happened, but it really seems like it could have been an honest (but definitely pretty stupid) mistake.
Edit: Not sure why this is getting downvoted, anyone care to explain? Action in startups is not always deliberate.