https://consumerist.com/2017/08/30/hacker-broke-into-hotel-rooms-electronically-stole-customers-stuff/

Key cards may be a convenient way for hotels to issue room keys, but a bug in one popular model made it convenient to electronically pick the locks. An override code to open doors was programmed into the locks, making them easy to open after a quick shopping trip to RadioShack. One man took advantage of this bug and used it to gain access to rooms across the country, stealing stuff from hotels and guests alike.

In an excellent feature story in Wired, you can learn the slightly horrifying story of how tens of millions of electronic locks in hotels have an easily exploitable flaw, but the manufacturer has no way to push an update out to all of them. Years after the flaw was discovered, many of them still haven’t been fixed.