I have a UK sim-free Samsung Galaxy S3. It's had official OTA updates only, and is not rooted or modified in any way. It's running Jelly Bean.
Since getting the Jelly Bean upgrade it's possible to get the phone to reboot by using cut and paste functionality. I've achieved this with both the Gmail app and Chrome. I've seen it on other people's phones too. I've performed a factory reset and the problem continues.
To reproduce, load up Chrome and then long-press on any webpage with text, select a little text, and then either press the highlighted text or the
copy to clipboard icon (two overlapping rectangles). After between 1 and 20 attempts, my phone will reboot. Sometimes it will reboot when I paste something; for example a URL into the URL bar at the top of Chrome's screen.
I've not been able to reproduce this on the stock browser, despite a number of attempts, nor on any other apps (apart from Gmail).
I'd be interested to hear if any other people are affected by this.