A past customer just called, with email trouble... suddenly their email had stopped delivering. This customer had been acquired by another company, and we had shut off their web hosting service months ago, as their website had been retired.
A few months after their hosting had completely expired, we cleaned up our name servers, removed a bunch of old zones that we no longer hosted -- or so we thought...
It turns out, this company was still using their old domain for email, and had never switched their domain away from our nameservers -- the acquisition had proceeded at a glacial pace. The Internet caches things for a long time, though, so it was some 6 weeks after we had removed their zone that mail had finally stopped delivering.
While he was on the phone, I was able to restore the zone from a backup taken two months ago and verify that the "MX" record now resolved to the correct place. We renewed the basic hosting account for another year, and the client was back in business! Yet another reason why we love having solid backup systems!