The latest version of my favorite hotel booking app, Booking.com, suffers from a bug that appears to be a typical “off-by-one”-error. As you can see in the screen shot below, the check-in and check-out dates may display one day ahead of confirmed dates.

More interestingly, the developers of the app made sure the current version first checks if there is a message, on the server, to be displayed when the app starts. That’s an excellent practice since it may take many days before Apple approves an update through the App Store. A message back door like this can be used for urgent bug notifications or even messages relating to forcing the download of another app (for example, if you need to change the publisher of your app).

Construct your back door so that you display text as well as tell the app what actions the user can take after reading the text, for example just OK, OK+More information (URL), or maybe just a Quit action.