The parents of redditor odin917 were away on vacation, and he needed to get into their mailbox. They had the only key with them. So they emailed him a picture of it, and he 3D-printed a copy of the key -- which worked.
As is obvious by now, leaving pictures of your keys online is a huge security risk, because it's so easy to turn a snapshot into a 3D model. But as Hackaday points out, this is the first time they've heard of someone using the technique for good!
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He did his modeling in Autodesk’s free Fusion 360 CAD software. He then printed it out, and the box didn’t open. It took three revisions before the perfect key popped out of the printer. This particular mailbox uses a 4 pin tumbler, which makes it a bit less forgiving than other mailbox locks we’ve seen