Well since it's under warranty, I wouldn't bother working on it, but...
It could be the throttle position sensor. Why? Because this tells the computer how much torque you want (e.g. how hard you're pressing the accelerator). If it has a dead spot, and you hit it just right, the computer might freak out like yours does.
I don't know if the GDI has drive by wire, but I'm assuming it does. So it could be the motor in the throttle blade. Why? Well this is ultimately what throttles the engine. So if it sticks, then yeah, this could happen.
Last thing could be the air idle control valve. Why? The IAC controls a small amount of air that bypasses the throttle valve. It does this to allow the engine to idle smoother and not stall after you take your foot off the gas. If the computer thinks your engine is going to stall, the IAC is where it usually goes for help first. I would never have thought it was possible, but the IAC valve can allow enough air to "get by" to rev your engine to near redline. I've had this personally happen to me (Well, it free-revved to 5,000 rpm) on a SRT-4 that I have. The car would drive itself at 65+ mph in 5th gear and my foot completely off the gas. New IAC fixed the problem -- I think the old pintle was wore out.
Again though, I wouldn't touch it though, it's under warranty. If you are really worried about it, you could try to capture it yourself by buying one of those tattle-tail OBD-2 recorders they sell for you to record how your teenage kid drives. Just leave it in there all the time and after it does it again, wait a few seconds and turn off the car and unplug it. Take it to your computer and have it spit out the last start-stop cycle and take that to the dealer. They should be able to see what happens if it logs:
- rpm
- %tps
- IAC movement
- vehicle speed
optional ones, but might help the dealer out more:
- fuel trims (to see why she started to stall)
- ignition advance
