alt Hacker NewsYou include it but with no mention of it. Allow recoding to be activated with an obscure undocumented button push.
Leave a subtle hong somewhere that someone clever can find out. Wait for news of the functionality to go viral and additional products to walk off the shelf bought by people who feel clever.
More likely to go viral in that case, a headline saying "Walkmans sometimes record over your tapes!"
When I was about 10, someone lent the school a tape of Holst's The Planets for a school play, one of the other students pressed the record button, and shortly thereafter the teacher played to the class a recording of me shouting "no stop" as I rushed for the stop button having seen what they'd done just a moment too late.
Now imagine that happening by undocumented feature, where nobody knows it would happen before it does.