There are many advantages to owning a jailbroken iPhone or iPad.
You can force apps to run full-screen on the iPhone 5, for example, regardless of whether or not they've been updated to support Apple's fifth-generation smartphone.
Unfortunately, some developers and publishers don't like you using their apps on jailbroken devices. Eidos-Montréal is one such developer.
Deus Ex: The Fall contains a pretty interesting anti-jailbreaking countermeasure, you see. Try to fire your weapon in-game, and you'll be presented with the following pop-up.
The above pop-up appears any time you try and shoot your gun, even if you've legitimately purchased Deus Ex: The Fall from the App Store.
Deus Ex: The Fall isn't the first app on the App Store to employ anti-jailbreaking countermeasures, mind. You can't use the Sky Go app on a jailbroken device, for example.
In other, slightly better, news, Deus Ex: The Fall will soon be playable on some older devices. Specifically, the iPhone 4 and the iPad 2.