Me too. There's a guy who sometimes fat fingers my email address instead of his own, over the years I've had a bunch of his receipts and confirmation emails.
It was a single letter, so I'd say it was a small one, but minor? Given the implication I'd say it's pretty far from minor. It's a typo that should've been preemptively avoided; all it took was the appropriate amount of caution and foresight. That it wasn't acknowledged as a problem immediately is astounding, but that it continued to happen for years without knowledge is most definitely unbelievable.
Making a typo that can send to the wrong place is a common error by anyone. Net security that allows it, presumably some of them from military intranets or in various correspondences without flagging it a problem, that's a huge mistake. The solution was a patchwork to make the problem a future one, which is so government typical. Probably would have required some reprogramming in COBOL, and they couldn't find anyone.