Q: I gave my sister my iPhone 3GS after i purchased an iPhone 4S. Recently I noticed that her calendar entries appear on my iPhone as well as her contacts. How can this happen? She must have her own iTunes account as I have never told her my details.
A: The most likely cause of this is that you did not fully wipe and reset your iPhone 3GS before giving it to your sister, and therefore some of the previous calendar and contact synchronization configuration remains from when you owned it.
If you were using iCloud and left your iCloud account configured on the iPhone 3GS then information that your sister enters will still be syncing to your iCloud account, and therefore by extension to your iPhone 4S. An iCloud account does not have any connection to the iTunes account configured on the device or the iTunes library that the user is syncing to, so even if your sister has synced her iPhone 3GS with her own iTunes library the old iCloud configuration will remain in place unless it’s manually removed.
In this case, the simplest solution is for your sister to go into her Settings app on the iPhone 3GS, select the iCloud option, and delete the account configured there. She will be given the option of keeping existing contact and calendar data, although she may still have some cleanup to do as she will also have inherited your calendar and contact entries in the process.
If iCloud is not involved here, the other possible problem is that your sister has at some point connected the old iPhone 3GS to your computer and iTunes has synchronized this information between your computer and her device in the same way as it would have when you owned the iPhone. Again, unless you’ve wiped the iPhone completely and set it up as new, iTunes will still remember the previous sync settings with your iTunes library, which may include contact and calendar info if this was the manner in which you previously synced your content when you owned that particular iPhone.
Keep in mind as well that with iOS 5 the iPhone can sync with your iTunes library over Wi-Fi, and will in fact attempt to do so whenever it’s connected to a power source. Therefore, it’s possible this could have happened even if your sister was simply visiting your home and plugged the iPhone 3GS in to recharge it while she was there.