That is a common misconception, but is actually not true. Source: I used to work at the Genius Bar, with a stint in APAC AppleCare Retail Service Operations, a

That is a common misconception, but is actually not true. Source: I used to work at the Genius Bar, with a stint in APAC AppleCare Retail Service Operations, and have confirmed this very question on one occasion with superiors in the US AppleCare operations team (= the people who come up with all standard operating procedures observed by the Genius teams around the world). That said, what is true (and may be misrepresented both by overzealous employees (one reason I went to some lengths to confirm it) and/or misunderstood by customers) is: - Apple Retail may (and often will) ask customers to restore their phone to a non-jailbroken state on their own (as opposed to doing it in store, for them) if they came to the Genius Bar for issues that have a potential root cause in software, and refuse service until the phone is reverted to a non-jailbroken state. - as with any intentional physical damage to the device, in the event that a software modification resulted in damage to the device that cannot be undone by restoring to stock iOS, Apple may choose to either charge you out of warranty repair/replacement pricing OR even refuse service. I am not aware of any jailbreak tweaks that would cause irrevertible damage, but I never dug deep into all the possible tweaks, have not jailbroken a device in years and am generally out of touch with the iOS ecosystem these days. Caveat: I switched jobs 3.5 years ago, so this stance may have changed. I consider it unlikely given my experience, but I am happy to hear evidence to the contrary :-)