I don't remember the details, but it probably didn't become misconfigured, it would always have been misconfigured, just one particular combination of factors that hadn't been seen before suddenly got caught. I remember at one point that an org I did email for couldn't receive email from a particular other organisation. Turned out that I had configured our MTA to accept TLS, and theirs presented an invalid cert, so the connection was rejected. This wasn't the situation I referred to, because this caused email to bounce hard, so people just routed around (yay for having people have professional contacts send to their hotmail instead).

Or whatever. I remember the feeling, not the mistake that caused it.