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Anybody like the OP shouldn't consider themselves a friend. They are letting you use their phone, why do you feel like you should be a jerk (I'd use other words here, but getting banned isn't on my to-do list today) and erase their phone? What if they wanted to call somebody, send a text, or do anything with their phone between that time and whenever they can next re-sync? Moving an app that they regularly use to another page is a harmless prank, but deliberately erasing their entire device and any changes that they made since their last sync is nothing that somebody that considers themselves a friend should do.

Thankfully I only have an iPod touch right now (and a 1st-gen at that), so people aren't really all over that anymore (which makes me happy), but if I had the latest iPhone I wouldn't be letting my friends use it to do anything other than check a website real quick, making a call or something small like that. I would have bought the phone for my own use, not so you can play games or check your facebook cause you're so addicted that you can't go 30 minutes without checking for notifications.
Dude, I said share your HARMLESS practical jokes. Do you know how much data could be lost if you smash a phone with a hammer?
He was being sarcastic, haha.
 
I'll lean toward the 2nd option. If you're going to play a childish prank, don't destroy anything in the process. That's just cruel.

+1. if i'm going to mess with someone i'm not going to erase anything they have. pretty much the only thing i've ever done is change the language on someone's phone but i made sure i knew how to change it back before i did it.
 
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