[nneonneo]

Rolling shutter won’t cause this: the readout of the whole frame happens in a fraction of a second, and I don’t see someone moving their arms quickly enough to trigger this kind of glitch. You usually see rolling shutter effects with objects that are moving very fast, like wheels and propellers.

[DennisAleynikov]

Yeah for sure, it would take a freak accident to move your arms so quickly and suddenly and have them end up sharp in the end result.

The closest I’ve been able to achieve in a few minutes with my phone is a blurry vs non blurry image of my hand in the reflection vs in real life. Pretty hard to pull off this kind of double exposure trick when the software isn’t actually intending to take double exposures

[nneonneo]

I wonder if this is easier to pull off with "night mode"? A few tests suggests that the iPhone is indeed quite choosy with the exposures that it stacks, so there's definitely a possibility of it picking-and-choosing parts of pictures to stack.

[justsomehnguy]

And perfectly stable hand what held that phone, not making a blur anywhere.

[grujicd]

Optical image stabilization can explain that part

[Xenoamorphous]

Is it possible that the phone was very busy/lagging and what would normally take a fraction of a second took a couple of seconds?

[nneonneo]

The camera hardware implements the readout upon command from the application processor (what you'd normally call the "CPU" of a phone), and the readout does have to be carefully timed in order to ensure a consistent exposure across the frame. So, whether the phone is overloaded or not will not affect readout speed (and therefore the rolling shutter effect).

This does not preclude the possibility that some other photographic process was stalled, such as HDR stacking.