I had an electronic module that worked in our lab, but failed at the customers, but then worked again in ours.

Turns out one of the pins in a space grade connector (MDM-25) open-circuited exactly between 68F and 70F. Our lab was 70F, and the customers was 69F. The way we caught it was to be watching the temp chamber when it slowly ramped from cold to ambient.

Turns out the connector supplier had shifted production to Mexico, and they were contaminating the contacts with RTV when sealing it.