Following up on my rant of a few weeks ago regarding unexplained and sudden correlated iPhone "bricking", the answer turns out to be more interesting than I would have thought possible!
It turns out that Apple iPhones 6 and later and Watches use an ultra low po wer 32kHz MEMS oscillator. Further, it turns out that helium in the atmosph ere can diffuse INTO the oscillator and cause the device to quit, at least temporarily.
It so happens that in this particular workplace, there are quantities of he lium vented periodically when the instrument on which we are working is coo led to from 77K to 4.2K (some leaks off during operation but it's relativel y slow). O2 levels are monitored for worker safety (and additional ventilat ion is employed when there is a lot boiling off) but helium levels are not monitored.
Further reading: ?Output Drifting of Vacuum Packaged MEMS Sensors D ue to Room Temperature Helium Exposure?
iPhone specific blog entry:
The part in question: